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AD-A016 091

LASER HAZARDS AND SAFETY IN THE MILITARY

ENVIRONMENT

ADVISORY GROUP FOR AEROSPACE RESEARCH AND

DEVELOPMENT

AUGUST 1975

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A 02A 27 LECURESERES o.7

goon

Laser Hazards and Safetyin the Military Environment

vo-

NORT''ATANTI TNATIONRALTCNICALIO

_____________________ 'FORMATION SERVICE _________________

DISTRIBUTION AND AVAILABILITYON BACK COVER

REPORT DOCUMENTATION PAGEI .Recipient's Reference 2.Originators Reference 3. Furujier Reference 4.Securit. Classification

AARL 9 of DocumentAGARW-LS-79 UNCLASSIFIED

r.Originator Advisory Croup for Aerospace Research and DevelopmentNorth Atla ntic Treaty Organization7 rue Aicelle. 92200 Neuilly sur Seine, France"- ~6.Title

Laser Hazards and Safety in the .M!itary Environment

7.Preentedat a Let ture Series in Germany (22 23 September 1975). The Netherlarnds(25 -26t September 1975) and Norway ( -"2 October 1975)

8.Authoms) 9. Date

Various August 1975

10. Author's Address I1l.PagesVarious 114

12.Distribution Statement This document is ostributed in accordance with AGA'!ADpolicies and regulations, which are outlined on theOutside Back Covers of all AGARD publications.

13. Key.woirsi/Descriptors Lasers Physics; 14.Ul•Eye (anatomy) Hazards Medicine 6'1.375.826:614.898:Ophthalmology Safety 612.014.48Irradiation BiologyReviews

I 5.Abstract

This Lecture Series No.79. on the subject of Laser Hazards a'.d Safety in the Military

Environment. is sponsored by the Aerospace Medical Panel D, AGARD. and implemented bythe Consultant and Exchange Programme.The Lecture Series is intended to provide an understanding (if the safety problems associatedwith the military use of lasers. The most important hazard is the inadvertent irradiation of

the eye and so the Series will include contributions from the physical and biological sciences,as well as from ophthalmologists. Those involved with las.cr safety come from many back-grounds - from physics to engineering and from vision physiology to clinical ophthalmologyand it is essential that each enderstands the contribution of the other.

The lectures include an introductory part and from this. the more advanced aspects of each-ubject are covered, leading to the issues involved in the design of safety codes and the controlof laser hazards. The final session deals with medical surveillance of laser personnel. TheSeries is of value to both military and civilian personnel involved with safety. whether theyare concerned with land. sea or airborne laser systems.

"M S13JRK TO OM

'1

AGAR-)L-S-79

NORTH ATL/.ATIC TRI-ATY OR(;ANISATION

ADVISORY GROUP FOR AFROSPACE RI-SEARCII AND •!.-LOI! NT

(OKCANISATION DU TRAIIE DI L'ATL.ANTIQL NORMD

AGARD I ett-ire St-.ics No.79

LASER [N.AZARIDS AND SAFETY IN TRE MILITAIC) N VI ' ON.WIENT

A

The material in this book has been assembled in support of a Lecture Seriespresented in Germany (22-23 September 1975), The Netherlands (25-26 September 1975)

and Norway (1 -2 October 1975) sponsored by the Aerospace Medical Paneland the Consultant and Exchange Panel of AGARD.

THE MISSION OF AGARD

The mission of AGARD is to bring together the leading personalities of the NATO nations in the fields ofscience avd technology relating to aerospace for the following purposes:

- Exchanging of scientific and technical informaiion:

- Continuously stimulating advances in the aerospace scienLe, relevant to strsngthening the common defenceposture:

- Improving the co-op.:ration among member nations in aerospace research and development.

- Providing so:ntific and lechnicAd adtice and assis!ance to the North Atlintih. Military Committee in thefield of aerc ipace research and development:

- Rendering xientific and technical assistance. as requested, to other NATO bodies and to member nationsin connection with res-arcl. and deselopment problems in the aerospa:e field:

- Pro•idinf, assistance to member nations for the purpose of increa:,ing their scientific and technical potential:

- Recom nending effective ways for the member nations to use thtir rtsearch and development capabilitiesfor thr common benefit of the NATO community.

Th-: highest authority within AGARD is the National Pelcgates Board consisting of officially appointed seniorreprtsentatives from each member nation. The mission of AGARD is carried out through !he Panels which arecompoied of !xperts appointed b% the National D)elegates. the Consuitant and Ex-hange Program and tile AerospaceApplicatins Studies Prograw. The results of AGARD work are reported to the :neinber nations and the NATOAuthorities through the AGARD series of publications of which this is one.

Participation in AGARD aztivities is b.i imitation oni% and is normally limited to citi/ens of the NATO nations.

The content of this publication has been reproduceddirectly from material supplied by AGARI) or the authors.

Published August 1975

Copyright © AGARD 1975

621.375.826:614.898:612.014.48

-?P

Printed by Technical Editing and Reproduction LidIlarford ltouse. 7-9 Charlotte St. London. WIP HID

ii

PREFACE

This Lecture Series No.74. on the subject of La.,cr Hazards and Safety in theMilitary E-nvironmnent is %l'oitiored by the Aerospace Medical Panel of AGARD. andimplemented by the Censultant and Exchange Programme.

The Lecturv S. ac• is intended to provide an understanding of the safety problemsassociated with the :rilita* use of lasers. The most important hazard is the inadserte- tirradiation o! the eye and so the Series will include contributions from the physical andbiological sciences. as well as from ophthalmologists. Those involved with laser saftycome from m.any bckgrounds - from physics to engineering and from vision physiologyto clinLal ophthalmology and it is essential that each understands the contribution of theother.

lIhe leo tures ;nclude an introduct*., part and from this. the more 'advanced aspect'sI h o , urci are covered, leading to •the issues involved in the design of safety codes

M:, .i.;ntrol of laser hazards. The floal "ssion deals nith medical surveillance of la,.r.;,.'. •onrcl The Series is of value to both military and civilian personnel involved with

sMetv. whether they are concerned with land. sea or airborne laser systems.

A V

Wi

i70

LIST OF SPEAKERS

Lecture Series Director: Wg Cdr A.N.NicholsonRoyal Air Force Institute of

Aviation M,'dicineFarnboroughHampshireUK

Prof. J.W.McGowarCentre for Interdisciplinary Studic-

in Chemical PhysicsThe University of Western OntarioLondonCanada

Mr D.'ISlin.y1Army asnvironmental Hygiene AgencyAbeideen Proving GroundMarylandUSA

Lt Col. E.S.Beatri.eDepartment of tl-e ArmyLaser Safety TeamLe~terman ArrmI)Institute of Res:archSan Francisco, CaliforniaUSA

Mr R.G.BorlandRoyal Air Force Institute of

Aviation MedicineFarnborough"Hampshire

UK

Dr D.ItBrernanRoyal Air Force Institute of

Aviation MtedicineFarnborough

HampshireUK

iv

2-1

CONTENTS

Page

PREFACE iii

LIST OF SPEAKERS iv

Reference

SAFETY WITH LASERSby A.N.Nicholson I

PROPERTIES OF ELECTROMAGNETIC RADIATION"by J.W.McGowan 2

"LASERSby J.W.McGowan 3

p ~INSTRUMiENTATION AND MEASUREMIENT OF LASER RADIATIONby D.H.Sliney 4

C(CULAR EFFECTS OF LASER RADIATION: CANCER AND ANTERIORCHA.MBER

by E.S.Beatrice

OCULAR EFFECfS OF RADIATION: RETINAby E.S.eatrice 6

DETERMINATION OF SAFE EXPOSURE LEVELS: ENERGYS•ORRELATLS OF OCULAR DAMAGE

by R.G.Borland 7

DERIVATION OF SAFETY CODES1. USA EXPERIENCE

by D.H.Sliney 8

DERIVATION OF SAFETY CODESI. UK EXPERIENCE

by R.G.Borbnd 9

OPHTHALMOLOGICAL EXAMINATION OF LASER WORKERS ANDINVESTIGATION OF LASER ACCIDENTS

by D.H.Brennan 10

LASER PROTECTIVE DEVICES

by D H.Sliney I I

BIBLIOGRAPHY B

k:V

SAFETY WITH LASERS

by

Wing Commander A N Nicholson OBE RAFRoyal Air Force Institute of Aviation Medecine

Farnborough, Hampshire. United IKintdom

The introduction of lasers into both military and eivil operations brought with it the problem of"the safe use of devices which emit beams of high energy light, and it was not very long before it wasonly too well appreciated that the eye was well equipped to focus the parallel beam emitted by a laserand so lead to aa important hazard. It is now realised that the early assessments of the hazard to theeye were exaggerated, but nevertheless the laser is a device which could scriously impair viaion ifadequate safety precautions are not taken.

Research directed to the problem of defining the hazart to the eye by high energy monochromaticlight and delineating the appropriate safety controls has betn an interesting example of many disciplitesdirected toward a common goal - safety with lasers. Biologitts with their knowledge of anatomy havedefined the changes in ocular structure caused by lasers which are either absc:bed by the t-ansparentmedia of the eye or by the retina. Biophysicists have defined the energy correlates of damage andphysicists have used these data to provide the rules of safety. The contribution of each disciplinehas been vital and the interaction between disciplines has been essential.

in many ways this lecture series will trace tne contributions of these separate disciplines to thesolution of the problem. In so doing the more elementary aspects of each subject will be carefullycovered and each lecturer will su=arise previous contributions escntial to understanding his ownsubject. This will help each student attending t*e series, whether from the biomedical or physical

sciences, to appreciate the importance of each contribution to the overall problem.

The initial lectures will describe lasers and their operation, and the nature of laser radiations.The lectures will also cover the physical terms used in laser work, which are essential to an under-standing of the problem, and, particularly, of measurement which is ixportant to adequate safety control.The next group of lectures will deal with the biological effects of lasers and the energy correlatesof dam;'ge to the eye and skin. These lectures provide the basis for safety codes.

The final lectures deal with the medical surveillance and protection of personnel at risk. This isan area of considerable importance - not least from the medico-legal point of view. Many doctorsinvolved in industry and in the armed services are faced with the problem of providing adequate medicalcover, and it is hoped that those respons'ble for medical supervision of workers in laser environmentswill add their experience to the series.

It is the aim of the series to provile a forum which will help to create a uniform approach tolaser safety within the NATO alliance. T) do this we hope that all member ciuntries will participateactively in the discussions.

2-I

PROPERTIES OF ELECTROMAGNETIC RADIATION

S~ J.Wm. McGOWAN

Physics Departmentand

Centre for Interdisciplinary Studiesin Chemical Physics

The university of Western Ontario

London, Canada N6A 3K7

SSUMMARY¥

Although the electroma'netic spectrum extends over smre than thirty ord-.,s of magnitude that portion of itnow dominated by th.. LASER only includes four. It is through this rtnge that all life processes areaffected by light, in particular the eye car. easily be damaged t-y 3,-t. In this lecture the basicprinciples dealing w-th electromagnetic radiation are discussed particularly as they relate to thedevelopment of the IVAER.

1. INTERACTION OF ELE,.TROMAGNETIC RADIATUI* WITH LIVING SYSTEM.From the beginning of time the interaction of electromaanetic radiation - light - with atoms, small

molecules, and eventually large biologically significant molecules, has led to life on this planet as weknow it today. Until this last century there had developed an equilibr.,um between the flux of radiationfrom extraterrestrial and from natural sources on the earth and with living systems. Now the ingenuityof man has led to the development of sources of radiation which range from home power frequencies,through radio, radar, infrared, visible, to x-ray which have significant effects upon life processes.Particularly dangerous is the n.•w light source, the LASER, which through the region of the clectro-magnetic spectrum which inc~udfs visible radiation cannot only disrupt biologically significant molecules*when the energy contained i. tne radiation is sufficient to dissociate or ionize them, but which cantransfer enough heat energy t. biological systems (most vulnerable is the eye) to literally cause them toboil. If sufficient energy iA deposited in the system in a very short time a mechanical shock candevelop which literally shatters the system much as the impact of a bullet on a window shatters the pane.

In this first lecture, I will discuss the entire electromagnetic spectrum with particular attentiongiven to that part of it that we can see, the visible region, as well as to that pa-t which embraces thefar red or infrared, the heat po.tion of the spectrum, and the far violet, or ultraviolet - the region"that we normally associate vjzth suntanning end skin cancer.

Although electromagnetic radiation of all frequencies falls upon the earth, the biosphere in which welive is shielded on the violet end of the spectrum from ultra-violet radiation by an ozone layer of theatmosphere which exists between 22 and 25 kilometers above the earth surface. Such shielding is now per-haps in jeorardy as a result of the pollutants dumped there by supersonic transports and frcon from spraycans. Similarly we are not boiled in our own juices, because of the absorption ef far infra'ed radiationby the water vapour in our atmosrhere.

Most vertebrates see radiation with wzvelengths between 380 and 700 nanometers (I fm=lO1-m=l milli-micron, 10 R) while the flux of radiation in which they .ive lies between 340 and llq0 nanometers (nm) .Some insects are sensitive to and can see all o' this ridiation. 4owever, we normally do not cons.derthat man can see in the ultraviolet and infrared, because of the absorption of theue radiations in thecornea and eye fluids. However, if the radiation is intense enough, not all of the radiation is absorbedbefore it reaches the retina. As a result, he can perceive radiation %ith wavelengths shorter than 300 nmand in excess of 1000 rim. This includes all of that portion of the electromagnetic spectrum where photo--synthesis and photobiolo.y take place.

It is not surprising that the powerful new light source, the LASER, has been developed through thisportion of the electromagnetic spectrum, since the atomic and molecular p'ocessec. which make possibieLASER action are the same processes involving rotational, vibrational and electronic excitation of atcms,molecules and ions, as are involved in life processes.

As we consider th2 radiation from various parts of the electromagnetic spectrui and the power avail-able from different sourf-es, it is important that all of us from many fields establish a comn referencepoint - since it is unfortunate that each field encourages a specixic set of units that best fits thecommunity. Man'7 of 0.ese are hybred and thus even more confusing.

Let me suggest that' the MKS (meter, kilogram, second) system be used. To facilitatc this, considerthe definition and equi';alencies for a few thir.gs:

Wavelength A of light in nanometers (nm), 10-9 meters is equal to 1 millimicron (mu)

or 10 Angstrum (2)

Energy E in joules is equal to 107 ergs.

£n.'rgy E in electron volts (eV) is equal to 1.6 x 10-19 joules23.06 kcal/mole.

Power J i.• watts is equal to joule/second.

2. ELECTROMAGNETIC WAVES

Wave motioi in a string, or the ocean, or a soundwave in air is generated by a moving (vibrating) ob-ject. Similarly, an electromagnetic wave, like any otler wave motion, is developed by periodic motion,this time -f an electrically charged particle, e.g., an electron. An electric field naturally exists

around an electron. A- it rives and its velocity rapidly changes, an oscillating elpctromagnetic field isgeneratea, and an electromagnetic wave is produced which has both an electric and magnetic componentt:rasverse to the direction of propagation of the wave. In Fig. 1, I show schematically an electro-magnetic wave pro) ai;atinq in the z direction with the vf-locity of light. The wave is plane polarizedwhere hoth the electric and magnetic vectors oscillate normal to one another and in phase. The plane of-olarization of the wave is characterized by the plane in which the E vector lies.

rX

Direction

#E Polarization

Iy

-S-

Figuve 1. An electromagnetic wave propagatingin the z direction an-a polarized inthe "-direction.

.e st-ec'.ra, of elect:oragnetic radiation is very extenri e, reaching frtxn e<trame./ long waves wh. h-. vc wa-,23,ngths that P:e thousands of kilometers long to very high energy ccv~eic rays with wavelengthst -j --h sr.-.ler than t-, diameter of a nucleus, 102-i. The notion o- a ciassice! oscillatron of charge as

. elect D.•gneL: wave generator breaks down as the wa..e]ez~th of the e-t.tte;l radiati wn ar.;roaches thes ze of tht- at', 0 .1 For radiatiot. which includ's the visible part of the ,,,ectr-:" we have to can-stder a atpmic or 3uant,--- oscillator governed by v-.ry special rules. Indeed U.SFRS oz-e based upon the--. antuT-, .ture of nature where waves are particlcE, that is, photons, and photons ere waves. For themcment -- t suffice to say that within the quantum picture the energy of the ph-con (a quantum of energy)is d4.e-tly proportional to the frecuency i of the oscillating charge

- n"

where the consgan- - :roportionality n i'- pianck's constant, 6.6 r iC- . joule sec.

Th: r~- tionsh.n nct--,en -he velocity of propagation of an electromagnetic wave in vacuum. -, and thefr*,er_- of the oscr':at.-tn v (Hertz, Hz or cycleý/sec) dnd the wavelength of the proprgated wave A

:he veloc:ty of liea.L c has magnitude of " x 10 m-/st... Althouoh all other waves require propagetion with-"* in a ..- dciu,-, elrctrorm.agnetic .aves propagate wi*rn a vacu•m wi-tn a constant velocity thr.-'ughout the

entire elaecro.manetic spectrum. However, if the EM wavo passes through a medium its velocity is changed.The rttio of the velocity c-f the electromagnetic wave in vacuu-m and that within the m-r4zum, v, is c=.onlyknown as the index of refraction of the medium

n--

The major part of the EM spectrum is shown schematically in Fig. 2, where we have listed the wavelengthin meters, frequency, in Hz (cycles per second) and energy of each photon in electron volts (ev), a unitprimarily used by the physics community to describe the energy of one electron which has passed through apotential difference of one volt (1 eV = 1.6 x 19l' joules). One cannot help but be impressed with theenormity of the spectrum which stretches over more than 30 orders of magnitude. Through this entire rangethe ;ame simple laws organized by Maxwell in the late -500's describe the entire electromagnetic spectrum.:;otice that out of the entire spe':trum the visible portion which largely governs life processes and visualcomnunication is very narrow indeed.

3. E..SSICti AND ABSORPTION OF PADIATION BY 4UANTUM OS'ILLATORS

By the tvrn of the century the stage was set for Planck and Einstein to roc.,gnize the importance ofthe quantum oscillator. In order to describe the distribution of EM radiation that was given off by hotbodies. Planck $ad to propose th.t the radiation that was emitted care in bundles of energy, quanta, in-steaj of cominn as continuous waves. Man finally recognized th- dual particle-wave nature of matter. Fora particular hot body in which radiation and absorption is in complete equilibrium, that is for a black-body radiator, Planck showed that for an infinite nu;-ber of quantum oscillators each with a differentfrequency v, the energy density of the raeiation between V and %J+dlJ is VV which for a svstem in thermale•u.librium at an absolute temperature T is given by Plancks law:

8 t h V ) @

cl [exp(hv/kT)-l]

2 -3

PHOTONENEROy FREQUENCY WAVEL.ENGTH

10vl0 1HZ 3u0~

10I 60Hz 5:10' 3ilO K0 -COMMER'IIAL POWER W6HzOdiol .0-4 IKHz 3 0' TELEPHONE

-. 3~ DIATHERMY (13.56 KHZ)-106- 1MHz 334,02 - --.. :=AM (535-1605 KHz)

10 31100 3 - DIATHERMY (27I2 MHO)

io~~' 10H asc TELEVISION S H

N00 INFRARED

I01. .10_-3X SYNCHROTRON OPTICAL10e 3:10 RAIT RADIATION

-3m ULTRA VIOLET

3 62 X-RAY$13MEDICAL X-RAYS

Figure 2. EVertrvm.agnetic spectrum showing the variou~s Spectral Regions.

Here k = 1.38 x 10-23 joules/0 K is Boltzmann's equilibrium constant. The expression states that there area8.u2 degrees of freedom in the syst em of oscillators with an average energy hvfexp(hv/kT) -11 per degree of

C 3

freedom at temperature TF.

If one considers a hole cut in the wall vf the blackbody zavity, the radiant pow' r emitted normal tothe emitting surface per unit area of the emitting surface per wavelength often cal'.ed the Aset-ralLradiant emittence of the blackbody can be expressed equa:.ly well in terms of a wa'-.:length interval betweenA and )-dA

W=,~ -C d)X watts/rn Am.

).5 [exp(hc/AkT)-Il

if the wavelergth A is given in nanometers (rnm) and C = 3.74 x 10O2 watts nm" /M2 . It follows then th~atone car define the spectcal. brightness of a source as the spectral radiant emittence normal to the emit-ting surface contained in a small cone or solid angle dQ steradians around the normal. This quantity isplotted in Fig. 3 for the blackbody radiator with a temperature which varies from 1000 K through to 10million degrees IC, a range which was unrealistic to consider at the time of Planck, but which now i-n-cludes the temperature of the corona of the sun, approximately 60060K1, the temperature for nuclear fusionabout 10 80 and the equivalent temperature of a high energy syncnrotron radiation source (radiationfrom highly relativistic electro~ns) approx~mately 10 million0 ?. I mention this latter source sincesynchrotron radiation sources which emit a very intense continuum from the infrared through to the x-rayregion are rapidly deve oping as research tools in many parts of the world.

From thLe Planck radiation formula it follows tn-At the wavelength associated with the distribution.aaximum 1. times temperature is a constant,

X T =2.9 x 10" nmrzI.

otnich is the well knewn Wien's displacement .aw. This rclation was identified empirically before Planck'swo~rk. in a similar way, Dne can derive the Stefan-Boltzmann law for the total power radiated by a black-body through the qurface of the area emitter summed over all wavelengths

=r j W(X,T'dX = GT'

0

most radiation emitters, with the exception of the LASER, are not as intense as blackbhody radiators,therefore t~e alackbodi curves represent the upper limits of powex emitted from a surface. many solidsand some g~as discharges radiate like an idealized blackbody. In iact, the spectral distribution emittedby inc~ndeseent lamps, and high densitly arcs and stars can be calculated to a good approximation fromPlanck's formula. As a reference pcizut, a blackbody at a temperature of 52000 ic has its radiation peak at

2-4

* I I I I

* ~sT ULTRAVIOLET --- -- INFRARED<- Ioe W'SIBLE

hr IIK

U) LAA-H HL SERSZ I-e

Cl Ioo_'0to

0n" WAVELENGTH (neI)

Figure 3. Spectral brightness for the blackbodl,- radiator as a function of temperature.

558 nm near the centre of t~he vi'sible spectrum, where the hum-n eye is most sensitive. Yet only 40 per-cent of the radiation falls within the visible pa.rt of the spectrum, six percent in the ultraviolet andthe rest in the infrared.

There is yet another 4uantum process which was important in establishing the particle nature of light-th.e photoei.e~tric effect. It was observed t~hat electrons were removed from a nretal surface only when theenergy of the photon was equal to or greater than the bindin.1 energy 6 of the electron in the metal,

.L (electron) =NhT - (.

Any excess energy went into the kinetic seergy KE of the outgoing electron. It is only sinee the adventof LASERS that it is realistic Lo consider what happer s when many photrns of insufficient energy to re-

lease an electrin arrive at the same time. N'ow ,-ultip~hoton excitation and ionization processes (that is,n.n-linear processes) are commonplace.

Once the concept of the quantud oscillator was recognized it followed directly that atoms withnegative electrons moving around postively charged cores did not continuously emit lightc Instead lightwas spontaneously emitted only when the alectron made a quantum jump from a higher level of the atom E2 toa lower one t (refer Fig. 4a). If hv equal to t he energy interval shines upon state t, the light can beabsorbed (Fin. 4b) thus exciting nre system, the frequenci of the light is given by

'J21 (E2 -EI)/h

$1i•NTNI•U$AQ,,"WW.PTION STOAUL.ATW(MISSIO EMISSON

E,

INCOHERENT COI•MEWTOUTMJ ouTWrT

(el(b) McFigure 4. Three iides of operation for the quantup oscillaton a) Sportcoaneous emission of a

l a oton of frequency V2a b) Photoawsorption and i) Stimulatod emission Of V21-

Rules kaor po selection rules govern the transition probability between States 2 and 1. T.e time onthe average it c e f th T u ansition to occur w s the radiative llfetdme of the excited syshea. Inthe case of molecules one m ust not only consider the aulectrontc trmitions but transitions; from one stateof vibration of the molecule to another, and a state of rotation of the molecule to another. The principalterms describing t thu e cing the system, inclutih electronic vibrational and rotational yergy are

EnvJ = (E. - j

n rv

where n is the electronic level, v, a particular vibrational level within the clectronic state and J nvtherotational sub-level. n

SP6MOSAOTO TMLU

EMSO MUO

2.5

it is important to heep in mind the relative magnitude of the intervals which exist between energylevels. Normally pure electronic transitions give rise to electro.-agnetic radiation which appears in the

k, :near infrared, visible and ultraviolet portions of the spectrum Tais corresponds to energies betweer, afraction of an electrcn volt to tens of eV. Pure vibrational transitions however occur in the red toinfrared region, while rotational transitions are dominantly in the infrared. It is these transitionswhich are the basis of radiation from LASERS.

When an atomic system is forced to make a transition from E2 to E2 (Fig. 4c) by light of frequencyV21, the light that is emitted tends to be in the same direction as that of the stimulating light so thatthe intensity of the emitted radiation adds in phase or constructively to that of the ztimulating light.It is just this process of stimulated emission which makes possible the formation of optical radiationwhich is intense, monochromatic, and in phase rather than being randomly distributed in time as is thecase when a number of quantum oscillators randomly decay in their own time. Such organized radiationsources have long existed ii the power, radio, television and radar portions of the E.4 spectrum but onlywith the advent of the LASER has it been possible in the optical part of the spectrum as well.

'4. LASER PROCESSES

Consider the usual relation which describes the attenuation of a beam of radiation passing through anabsorptive medium. This is the familiar exponential relationship (Beer's Law)

I(X) = I exp(-Cgx)0

where I(x) is the intensity at a distance x of a light beam originally of intensity 10 after passingthrough the optical medium of optical thickness cix. Oi is the absorption coefficient, which can be writtenin terms of the Einstein coefficient for the absorption of light, BI 2 , and the stimulated emission oflight, B21, simply

ai , NIB12 - N2B21

where Ni and N2 are the number densities of atoms in the lower state I and the excited state 2. Since theprobability of absorbing the radiation or stimulating its emission are equal, B12 = B22 = B. it followsthat

c= B(W2 - N2).

Anyone from 1917 onward could have readily observed that Ct can be made negative if N2 is greater thanN thereby causing Iix) to grow larger than I the original intensity as x increases. This possibility iscalled negative absorption or amplification. °In other words, amplification of the radiation only occurswhen the number density of particles in the higher lying excited state 2 exceeds that in state 1. Thissituation constitutes EoZulation inversion. Although the process was extensively studied through the 20'sand 30's the inventirsi of x-he LASER, light amplification by stimulated emission of radietion bo°'h as alight source and light amplifier did not occur until the late 50's.

Ther-t are many ways of establishing a population inversion in gases, liquids and solids. This will bediscussed in lecture 2 along with more details associated with various types of LASERS. For the momentlet it suffice to say that for laser action to occur an amplifying medium must be artificially producedand that this medium must be set in an optical cavity (Fig. 5a) bounded on each end by mirrors, one ofwhich is 10 - 90 percent transparent. This will allow the stimulating photons to resonate manv tiaesthrough the medium in order to cause the maximum Lllowed depletion of the excited atoms. The radiation

S - ill be amplified (Fig. 5b) on each pass through the system as long as the system remains irn an inverted"- state. If the gain per pass of the radiation is greater than the total loss per pass, then the system

will be made to lase.

OPTICAL CAVITYI 1i- . ... L .

N M

INVERSION Ilr

Figure 5. Shown in a) is the optical cavity with Figure 6. spectral line centered at v2 1 showingthe medium slightly excited. In b) seven rnrmal cavity modes with in-there is a marked population inversion. tensities which exceed the thresholdSome levels are stimulated to emit. for laser oscillation.

Since normally each atom prefers to be in its lowest energy state, an external source of energy is re-quired to maintain an inverted system. It is not sufficient that this external energy source s' .ghtlydisturb the Boltzmann (thermal) equilibrium, it must develop population inversion. This process is calledpumping. As long as pumping continues the inversion is maintained. If the pump should be stopped theatoms will rapidly return to an equilibritim between states through the process of spontaneous and

stimulated emission, and lasing action will cease.

We have seen that the frequ-ncy of the laser light is limited to narrow band centred around V2 1assocated with the spectral width of the transition 21. This width includes the width due to the naturaldecay of the excited state, the motion of the radiating atoms and pressure broadening. However, withinthis ioroad band of frequencies the LASER radiation is even more restricted by the properties of theoptical cavity. Atoms which oscillate in phase with one another in the cavity are said to be in normalmodes. The frequencies of the normal modes of the free oscillations &re harmonics of the fundamentalfrequency V2 3, Fig. 6. Within the optical resonating cavity of length L, standing waves similar to thosein a string occur only for wavelengths which are an integral number of one-half the emitted wavelength A,so that

L = 1/2 m = 1,2....

From the simple relationship between the velocity of light, wavelength and frequency discussed above, thefrequency interval between adjacent modal lines is

V= c/2L

If for examfle for red light where V21 is approximately 5 x 10 Hz (as for He-N 2 red laser light2 and anoptical cavity of length 1 meter, then the number of modes that exist in this case is 3 x 10& In otherwords the radiation which had a line width associated with the atomic transition plus doppler shiftingplus pressure broadening is now divid-d into nearly three million parts only some of which will show LASERaction because they meet the necessary inversion criterion. The spectral width of each of these linesassociated with the normal modes of oscillation of the cavity is at least a million times narrower th.anthe original spectral line. It is reasonable then to imagine that in the case of 5 x 101, Hz radiationwith a normal line width I-0 Hz, Fig. 6. the line width associated with the excitation of a simple modecan in principle if not easily in practice be made 1 Hz, thus the spectral brightness or the amount ofpower available p.r unit area of emitter within one steradian at a given wavelength or frequency is ex-tremely large, in fact larger than any other source.

N.ormally we also consider the spatially distributed modal 1.attern from a cylindrical or a rectangularcavity. This pattern is quite complex containing many transverse electromagnetic modes TENm the detailsof which are beyo:,d the scope of this lecture. In the designation of modes m and n are integralvalues where fcr circular mirrors n denotes the order of angular variation and m the order of radialvariation. :EM 0s usually the dominant mode in most cavities.00

5. PR•CIERTIES OF LIGHT SOURCES

The special properties of the radiation produced from laser action will become clear as we compare theLASER as a light source with other sources of electromagnetic radiation:

a. Point sources and extended sources - Although all light sources have finite dimensions it is use-ful idealization to consider a source as a point, even though there is no true point in nature. Forexample, an atom has its extension and sta,.s ".'hieh appear to us as points in reali.ty are very large. Thelighi from a noint source differs from that of an extended source in that it propagates radially from itsor:gi•n. Close to the source most of the rays intercepting the small surface area A strongly diverrje, how-ever as that same surface area is moved off at a large distance, divergence is minimized and the light canbe considered collimated.

An extended source by contrast can be considered as made up of a large number of point sources. Closeto this source, the light rays passing through the test area A have a larger divergence than those from apoint;: however, as A is moved off to a very large distance, often referred to as infinity, the light be-haves like it ccoes from a point source. Unlike most other extended light sources the LASER because ofthe organized nature of its radiation can be considered as a point source, even though in reality it isn=t.

b. Monochromaticity or Te=peral Coherence - A few years ago one would have called the light from amermc:z- arc lamp monochro=ietic. However, when this light is viewed through a spectroscope one finds itmade up of approxim.ately five l1nes with the dominant line in the blue. Since the advent of the LASERthe spectral width of the blue line is reduced by more than a million so that the light for the first timecan truly be considered monochromat.ic.

c. Spectral Coherence of Light - Light from a point source has a very special quality. spatial co-herence. If light could be emitted from a point source, anywhere on a sphere surrounding the source, theelectromagnetic wave would show the same maximum or minimum in its intensity. This light is coherent. Asone backs away from the point to infinity, the light reaching the observer remains in phase or in step.

In the case of LASER the very process of stimulated emission which produces the amplification of thelight leads to the emission of radiation in which all the waves moving in one direction are in step or inphase, quite similar to the situation one observes from a point source at infinity. The coherence of laserlight thea. is one of its most important properties.

The coherence of LASER light is best observed through interference and diffraction effects. They in-volve the constructive and destructive interference interaction of the electromagnetic waves. InterferencLeffects can most clearly be demonstrated with monochromatic, coherent light from a LASER. In fact, inter-ference photography, holography, is onlj realistic with the LASER as a light source.

Consider the case of diffraction from a narrow slit of width d. In Fig. 7. I show monochromatic co-herent liqht coming from the left. illuminating the slit. On a screen some distance away is the diffractionpattern. The position of the maxima where there is constructive interference of the waves is given by the

2-7

Figure 7. Single slit

diffractionTIC pattern

simple formula= (d/2)sin 0

where n (known as the order) is the number of full wavelengths that a wave coming from a pLint source inthe m-iddle of the slit is shifted so that at the screen it constructively interferes with one from theupper or lower edge. e is the angle between the direction of the collimated rays and the point of obser-vation on the screen. From an examination of the figure, one can readily see that if the light wej • notcoherent at the slit (that is. the waves were out of phase), and many wavelengths were involved, nodiffraction pattern could be recognized at the screen.

d. Polarization of Light - Electromagnetic radiation may be polarized in a number of ways. In thecase of radiowaves which are produced by the motion of electrons up and down an antenna the wave is by

ture ized In Fig. 1 this would be along the direction of the E vector. Normally light which isgenerated by very many atomic oscillators each acting independently is not polarized. Hr-ever, it chn bemade so by reflection, as for example reflection of sunlight at an angle near 500 from tA.u- surfac of alake: or the back scattering of sunlight from the molecules in the air. Such scattering is known asRayleigh scattering. Within our common experience we find that polaroid sunglasses eliminate the enormou-glare associated with such processes.

7!'e radiation normally obtained from a gas LASER is also polarized, not because of some basic atomic

process but because the exit window of the LASER is mounted so that the beam axis is approximately 500

with respect to the normal to the exit window. At Brewsters angle only radiation which is polarized will

build up within the laser cavity. As a result the light which is emitted is highly polarized.

6. GENERAL DEFINITICUS AND COMPARISONS BEThEEN LASERS AND OTHER SOURCES

LASERS vary considerably in output power from a few thousandths of a wat* as in the case of the veryuseful (red) helium-neon gas laser to the order of terawatts in the Q-sw.itcheq (fast pulsed) carbon

•ioxide-gas (infrared) laser. Some LASERS are capable of operating continuously (cw_ while other types of

LASERS are operated in a pulsed mode. In the discussion which follows, we will ce'pare a helium-neon cw

4as laser with a power of one milliwatt with other light sources.

a. Divergence and diffrdction limit - Because of diffraction any bear. of lig..t emitted from a sourcewith a small cross-sectional area diverges with a minimum half-angle of divergence 9 given by the rrtio ofthe wavelength to the diameter of the beam, A/d. This follows directly from the single slit diffractionequation discussed in the previous section. A beam of light having this d.vergence is sai4 to becollimated to within the diffraction limit. Once again because of the monochromaticity and coherence ofLASER light its divergencc is ideally that set by this limit. On the other hand light from other extendedsources, because the light is neither monochromatic nor coherent, has a divergence that is considerablylarger. One of the best and most dramatic examples of this is in the now classic picture of tLe partiallyeclipsed earth as seen by the T.V. camera of surveyors where two low power argon ion laser beans (in thegreen) can be seen eminating from a region in the western part cf the United States while none of the lightfrom the major centres in the West can be seen at all.

The approximate 600 nanozeter red light from helium-neon LASER with the output diameter of 2 um has ahalf-angle divergence 6 approximately equal to 3 x 16-" radians, or 0.0167C. Consequently. one canconsider the beam produced by a point source located some distance behind the exit mirror of theoscillating cavity. The laser equivalent point source radiates energy only within che cone 26 while thebrightness of this extended source is very high in the direction of the beam it is zero at other angles,which are outside the cone 20.

b. Radiant Power, radiant emitteuice and intensity - A point of coherent source is measured by itsradiant power, the measure of energy it emits in a unit time in all directions. Radiant emittence is theradiant power emitted normal to the emitting surface per unit &rea of the emitting surface. The intensityis the radiant power per unit solid angle.

S. . . . . . ii 1 1

Consider a sphere of radius R around a puint source* which has some closed area on the surface, as showri in

" ig. 8. The area on the surface divided by the radiusa of the sphere is the definition of the solid angle(Q = A/R2 ) measured in steradians (Sr). Since the entire

surface area of the sphere is O1R2 it is clear from thedefinition that 4V steradians represent the maxim- solidangle around a point source. If we consider a fixed arealike the size of the cornea of the eye, as one moves awayfrom source the intensity of the light which enters theeye decreases as h/R2 since the solid angle subtendedfrom the point source changes as 1/9 2 . This is what isnormally called the inverse square law which governs manyof the basic physical principles of natare.

It can be easily demonstrated that for the LASER,the solid angle into which the power is emitt ed from apoint source is equal to VL2. Since for our He-Ne LASERd is equal to 3 x 10-" radians, and if the power is Imilliwatt, it follows that

0-3Inte-nsity 1 4 x 10 watts/Sr.

Figure 8. Sphere showing the elements S t3 x 1

defining solid angle.

c. Brightness - .. " brightness of an extended source is t.e raliant/emittence/unit solid angle or theintensity/unit area of the e.'itter. Bot- intensity and brightness fall off as cý" the angle with respectto the nornal to the emitting .zurface. I f in the case of the I milliwatt LASER the radius of the beam isI = or 0.001 meters,

Brightness (LASER) 4 x 103 watts/Sr 3 x 10 wattr/m2/Sr4--- (0.001)

2m"

By comparison. for various other light sources:

Brightness (Tungsten filament @ 3300°K) - 7 x 10 watts/Mi /Sr

(High power carbon arc) - 3 x 10' watts/m2 /Sr

(Sun) = 2 x 10' watts/m2 /fr

(0.25 M(W Synchrotror, radiationsource) = 3 x 109 watts/n /Sr

Thus, the smallest of lasers, is bright.r than all known light sources. in fact it is two orders afmac.nitude brighter than the sun, the source of all life on this planet.

d. Spectral Brightness - We have already spoken of spectral brightness 4s it relates to a blackbodyradiator. Once again it is defined as the brightness of a source per unit wavelength or frequency.

There is no comparison between the Arlount of power in the form of light which can be delivered from --HIe-;e I milliwatt LASER and a 104 watt carbon arc radiating in the same small slice of the totAi -.. sionspectrum. Although 10 million times more total optical pvwer is delivered from the arc the amount ofpower in a small spectral band is much larger for the helium-neon LASER. For the arc lamp the powe. isdistriuted over approximately an interval of 500 nanometers, therefore, we have 10,000/500 - 20 wztts/nm.However, in the case of the helium-neon LASER, which as we pointed out before could be made tc. have afrequency wif'th as narrow as one Hertz by the choice of a single mode (I Hz at the wavelength of theheliu•-neon laser 600 nm Lnrrer-onds approximately to a special line width of 1 x 10"2 ni), the power perunit wavelength is equal to 160" 7/30- or 10' watts/nm. In other words the amount of light available in anarrow spectral band is much larger from a laser than from any other light source.

e. Illumination at a distance - Although it is possible to photograph the light from the argon-ionLASER on the noon, is it realistic to imagine that lasers can be used tn light the surface of the moon?Let*s look at this problem. A distant surface subtends a very zmall solid angle at the source. Thereforeone wants a source that emits a great deal of light into a small solid angle, and that is just what aLASER does. Let us compare the amount of light which the 1 milliwatt LASER can bring to a distant surface,A, with the amount coming from the same surface from a Tungsten filament at 100 W incandescent lamp.Since the lamp radiates 100 watts into the entire si.nere of area 4 R the power reaching the area A on thesphere is given by

Power - Intensity x Solid Angle

Power (Tungsten Bulb) = 100 (Watts) x A (Sr)

= 8 A/R 2 watts

distributed over the entire visible spectral range. From above,

Power (He-Ne Laser) 4 x 10 3 wats/Sr x - (Sr)

4 x 10' A/R2 watts.

* ;.9

in a single spectral line. It follows then that the ratio of the powers reaching a small area A is

Power (He-Ne Laser)A

Power (Tungsten Bulb) 0A

When one remtmbers that LASERS have been developed that are a thousand-million-million times more in-tense than our heliun-neon laser one xecognizes the enormous potential for the transfer of energy andinformat

4.on available through the LASER.

In Section 3 we showed that the brightness of even the smallent helium-neon LASER is in excess of thatof the sun. Doer this mean that the LASER rnaced as far away as the sun could do a better job than thesun in illu=inating the earth? Of course not! The power of each is its brightness times the solid anglesubtended times the area of the emitting surface. Under these circumstances one sees that

Power (Sun) 2 x 10' watts/m2'Sr x -(101Y x A/R

2 (Sr) 6 x 1025 A/R

2 watts

Power (Las,.r) 3 x 10e watts/m2/Sr x '(1C

3)

2 x A/R

2 (Sr) 9 x 102 A/R2 watts

7 x 1022

Even though the sun is a source of lower brightness than the LASER its very large area more than makes upfor it.

f. rcncertration of pov'er into a small area - Though it won't be proven here, radiant power densitat a point or. some area which is being illuminated by a source depends only upon the brightness of thesource. In this case the size of the source is immaterial. Furthermore, the. power per unit irradiatedarea has a value which is the same order of magnitude as the brightness. Since the laser has the greatestbrightness of all light sources, it follows that the laser is capable of producing a greater power densitythan any other sources.

As one zight expect the smallest area into which radiation in a parallel or nearly parallel beam can

be focused by a lens is limited by diffraction to an area of approximately A where ). is the wavelength ofthe radiation. The highest power density produced by 1 milliwatt LASER is thus given by a power outputdivided by ý

2 or

Power Density He-7Ne (ASER) 0 watts 3 x 10s watts/= 2

(600 x I0- 2

Note that the value for the power density is within an order of magnitude of the brightness of theLASER. Reme-eer again that this particular LASER is one of the lowest power LASERS. Therefore, as oneright expect the effectiveness of more intense LASERS like a 6000 watt CO 2 cw LASER for the machining of=etals, welding and other such purposes, is extremely good. Another impressive example is a picture ofapproximately ten burns in one hemcglobin cell caused by the light of a ruby LASER focused onto the cell.Microsurgery using LASERS is now a reality.

Although LASEPS are far better than any other =an-made sources for many purposes, they are not thesolution to all problems. Other light sources are far superior to the LASER for many purposes such asgeneral illumination. The anmlications of LASERS will be discussed in the next lecture.

7. GEJ-VER'L RFERENC~r

Allen, L., £ssentials of Lasers. Pergamon Press: Oxford. 1969.

Goldman. L. and R.J. Rockwell, Jr., Lasers in Medicine, Gordon and Breach, Science Publishers, Inc.:N.Y., 1971.

Halliday, D. and P. Resnick. Physics, John Wiley and Sons, Inc.: N.Y., 1962.

Laser and Optical Hazards Course Manual. U.S. Army Environmental Hygiene Agency: Aberdeen Proving Ground,

Md., 1975.

Laser Technology and Applications, edited by S.!. Marshall, McGraw-Hill Book Company: N.Y., 1968.

Lasers and Light: Readings from Scientific American, introduction by Arthur L. Schawlow, W.H. Freemanand Co.: San Francisco, 1969.

Lengyel, B.A., Lasers, Wiley-I:.terscience: N.Y., 1971.

8. AC10;OWLEDGMENT-S

I gratefully acknowledge the assistance of my associates P.K. John and J.A. Kedeiros. Physics Depait-"went, The University of Western Ontario, who assisted me with the literature and reading of the manuscriptsand the help of D.H. Sliney, U.S. Army Environment Hygiene Agency who brought the Laser and Optical CourseManual to my attention and the support of the Canadian Defence Research Board and U.s;. Army MedicalResearch and Development Command who with the Richard and Jean Ivey Fund and the Veeco Foundation aresupporting our research.

* f~ZW3-I

LASERS

J.Wm. McGOWANPhysics Department

andCentre for Interdisciplinary Studies

in Chemical PhysicsThe University of Western Ontario

London, Canada N6A 3K7

SwMARm

Princirles and properties of the LAS•FR are discussed in some detail together wlth. a description of thevarious types of LASERS and their applications.

1. MORE ABOUT LASER PROMP1TIES

The acrcnym LASER starnis for Light Amplification by Stimulated Emission of Radiation.

Several years after the Russians Basov and Prokhorov i and the Americans Gordon, Ztfiger and Townes 2

had shown that stimulated emission of radiation at microwave frequencies could be accomplished with theammonia molecule in preselected states, Schawlow and Townes3 proposed that such amplification could occurin the Infrared and throughout the entire optical region.

In order to maintain amplification, we mast cause the system to be sufficiently excited so as to havea net round trip gain for the radiation in. the laser cavity at least equal to unity. However for oscil-lation to build up over and above the spontaneous emission, the net round trip gain must be greater thanunity. This leads to the development of the minimum inversion necessary for laser action. The relation-ship describing this is known as the Schawlow-Townes criterion. If one assumes that the shape of theradiation line is Lorentzian (somewhat bell shaped as in Fig. 6, Lecture 1) it follows that

N2'_- NJ > 87 c AV _ =••

where Nc is the critical population inversion density necessary to maintain laser oscillation. In the ex-pression N2 and NJ are the population densities for states 2 and 1 while 92 and g! are the degeneraciesassociated with each of these states. Av is the half width of the broadened spectral line, while T2 isits natural half-life and Yc is the lifetime characteristic of the cavity construction. One sees im-mediately that as the energy interval between levels increases, that is. as the waveler lth decreases, thenecessary critical population inversion density increases rapidly. Similarly as the lifetime of theradiative state increases so too does the necessary critical populatiton inversion density. However, thereis advantage in using states for the upper laser level with very long lifetimes, since atoms decayingfrom still higher levels may be trapped in the lasing level, thus increasing the density of excited atoms.

Many materials can now be made to lase, not only in the infra; .. but the visible and near ultravioletregions of the electromagnetic spectrum as well. There appear to be serious limitations to developing afar LV or x-ray LASER but serious study suggests the y-ray lasers may yet be developed".

Using the above mentioned criterion for minimum inversion LNc necessary for laser oscillation, for aruby system which will be discussed in greater detail later one can estimate that the ruby rod 10cm inlength can be made to amplify radiation if the population of the upper le~yel exceeds that of the groundlevel by as little as 0.7%. Since ee concentration of chromium ions Cr3 in pink ruby is 1.6 x 1019atoms/cm3 , states I and 2 each contain approximately 8 .. 10l atoms/cm3 , and the population difference

- must be approximately 5.6 x 1l0 ato=s/cm3 .

The formulation described above was developed in assi-ciation with a model two-level laser system; how-ever, most solid state LASERS like the pink ruby system are three, while most others are four level LASERSlike neodymium doped systems. As a result the simple criterion is only a rough approximation. Exactanalysis of a system requires that one solve a series of rate equations associated with the total numberof excited atoms in which the ratio of populations in various states under stationary conditions can bederived. If indeed the system is excited by a blackbody radiator, one then can also calculate the powerof optical pu-mping radiation necessary to cause and maintain an inversion. It follows then that one cancalculate the equivalent minimum source temperature capable of producing adequate illumination in thespectral energy interval where, for example, the ruby absorbs. For .:he ruby rod disu-ssed above the nec-essary blackbody temperature of the pumping liqht source must be at least 3300eK. In reality it must behigher because of other complicating factors. However, this is realizable today.

The primary method used for laser pumping is the intense flashlamp (Fig.1) which normally is mountedalong with the laser cavity in a geometrical system of mirrors that effectively focuses all the opticalenergy on the solid, liquid or gas in which the inversion is to be produced. Often inversion can be pro-duced in gas discharges through any one or more of a number of processes which include direct and reson-ance excitation, energy transfer and dissociation. In what is now known as a CHEMICAL LASER, inversion isbrought about through the chemical reaction involving several atoms and molecules or in ion systemsthrough electron excitation or charge transfer. In semi-conductors, the applications of strong electricfields across the junction can cause inversion as can the bombarding of the sem.-conductor with an ex-ternal beam of electrons. Similarly in high-pressure gases high power in the visible and near ultra-violet can be pxoduced with the aid of a very high energy, high current electron beam.

2. NOIES OF OPERATION

Many LASERS, particularly low powered LASERS can be made to operate in a continuous wave (cw) mo.ewhere power is continuously added to the systmg, to maintain the inversion at the same time light i.n ex-tracted from it. As more power is extracted from the system the light will tend to oscillate. 9he&at-xal pulsation reflects the repeated breakdown

3-"

natural pulsation reflects the repeated breakdown of the minir.um inversion criterion for laser oscillation

caLsed by the depletion of the excited state due to laser action, or focusing effects on the light beams

due to the chance in the optical properties of the medium during operation. These regular pulsations of

the LASER are disturbing for most applications, especially corounicatien -where the timing and the control

of the intensity envelope are particularly important.

Often LASERS are intentionally worked in a pulsed mode. This is brought about by pulsing of the

optical radiation or the discharge, or the electron beam or tue electric field. The length of the light

pulse from the laser may not correspond to the length of time the exciting radiation is on since once the

system begins to lase often inversion can no longer be mainta.ned. Also, any change in the focusing

properties of tke maediu- can cause oscillation to be quenched.

Since the LASER is an oscillator, consisting of an amplifier with a feedback device, the threshol2 con-

dition of oscillation is reached when the gain of the amplifier is greater than the sum of the losses. The

loss rate of the system i6 frequently des-

cribed by the quality factor s. As Q de- S C 8

creases the loss increases, therefore the "-t•cnecessary for the oscillation to begin also

increases. Because of this relationship, the

techniquje us-'d to hold back the onset of os-cillation by temporar-ly increasing the losses

:n the L RSER is -,llfed "i-swtichin or 9-

S-w)l!lnc. For efficient production of a

single giant culse, it is essential that the

•'-sw-tching process te fast in comparison to %4-the lifetime of the pl.rto. within the cavity,

hence the time of switchin; between low and

high 9 can be chosen so as to assure the

development of the greatest poss:nle ia-

version in the material. -- •

In general the principle of 9'-switching consists one of many laser configurations. The

of inserting a switch (Fig.2) into the laser cavity. Figre 1. laser cavity is usually defined by

This switch can be activated during the pu;.1 pulse mirrors at each end, one of wnichin such a way that it separates thc ptrnping properfrom the laser action. The s semi=transparent.cavity can then be made very high to be released in

one single giant pulse. Since trfm-en&dis loss occursS~~~~~during pum•ping, the tot'al output energy released •'•• -

during tne ý,-suitching m.-ode is normally less than,0: that of the normal mode of operation. However, sinceth•e i-ilse is very short t-he power output is trem- fl

endously incre-ased. For example, values in excessof :erawatts'1oi watts) correspond:n-; to an energy U

4release greater than 100 joules are now common. R M "

McClung and K_-llwartn15 in 1963 wcre the first toproduce grant pulses with the rutv LASER. They usedan optically active Kerr cell as a shutter, making Figure 2. Sche' atic representation of a 9-switcneduse of the preferential ;olarization uf the laser laser system. In some systems the switchlight and the rotation of tize plane of polarization and mirror are combined.of the light when the Kerr cell was activated.

Mechanical switching has existed from th-- very zeginning. The idea of employing a rotating chopperwneel to open and close the optical path between the active -ediun, and one of the mirrors was first usedby Collins and Kislik< in 1962,s however this m-ethod Jr inherenrly slow. Tens of microseconds have elapsedfrom the time the chopper slot first begins to expose the active mediu until the medium is fully exposed.Faster mechanical switching may be accomplished by rotating one of the mirrors, or by replacing one of themirrors with a rotating total-reflecting prism.

: much sl--pler, and more effective 9-switch uses a dye solution which bleaches within ns when the im-pinging intensity surpasses a minimum value and beco-.es corniletely transparent. The dye must have twoenergy levels, which are separated by an energy equal to that of the laser photon. When the laser lightis absorbed by the dye molecule, all electrons from the lower level are lifted up to the higher one. Whensaturation is reached, that is when the louer level is completely emptied, absorption of the radiation inthe dye ceases suddenly, thus rapidly changing the Q of the cavity. Phthalocyanines are coonly used asdyes, the sane dyes that are used in inks'. Saturable absorbers in glass (CuSO RGB-Schott Filter orVPAUYL in Corning 3-78) can also bi! used; high lower densities tend to damage the shutter.

3. FP.EUEXE"CY TU;ABILITY

From its very nature the LASER stimulates the emission of the radiation associated wit? the transition

between the higher and lower lying states of the atrn,, ion or molecule in the resonant cavity. Althoughone frequenc, may be the dominant frequency the following proc.esses give rise to some variation in thefrequency of the radiation emitted from tne LtSER, and its associated optical system:

a. Modes of oscillation - As demonstrated in the previous lecture, within the natural spectral :)and

width of the emitted radiation, the It.ysical dimensions of the optical cavity will cause a very largenumber of standing waves or modes of oscillation to exist in the cavity. Not al. of then will be causedto oscillate if for no other reason than the inversion -ond-4tion is not met for all of them. However,associated with each spectra; line there are a limited number of modes through which the laser can betuned. in the ruby LASER, as in the He-Ne case each of the nodal lines m.y be as narrow as 1 iHz.

S3-

b. Sti•nulatec* Raman [SLissione - When for example ruby laser light at 694 nm (6943•)from a millionwatt pulsed LASER exc.tes complicated mol-.ules sucn as nitrobentene. or in fact any solid, liquid or gas-ithin or even outside of the laser cavy this leads to a number of spectral lines that are shifted1-rwn in frequency from the original lir . amounts corresponding to the electronic, vibrational orrotational levels of the molecules with.., the material which is being bombarded. This is known as RamanEffect. Whe.i the light intensity is very h:gh as in a laser cavity, the energy available as Raman scat-tered light is mary order•s of magnitude higher. TIis is the stinulated Raman effect. For example, if ina focused light experiment the electronic, vibraticnal or rotational levels of the molecules in themedium are stimulated bv the Raman effect, then the refractive index of the ma'erial oscillates strongly

- at one of these frequen.ies. As more light is add(d to the system the characteristic frequencies of themolecules are not simply subtracted frcm the origiral laser frequency they are added to it as well.These shifted frequencies are called upper and lower side bands, each line of which is a source of light

•€ at a different frequency.

The large variety of substances showino the Ranan effect provide hundreds of new coharent sources oflight from the ultraviolet through to the infrared. As much as 20% efficiency has been obtained in con-verting laser light to its harmonic frequencies, b:y means of non-linear processes, such as Raman scatter-S~ing.

c. Frequency doubling, tripling and quadrupling' - Other non-linear processes in certain asymmetriccrystals such as calcite, and potassium dihydrogen phosphate (KDP) can lead to the conversion of up to afew percent of the eneray contained in a laser bea-n of frequency v into frequency 2v. This is comonlyknown as frequency doubling. and is one of the prinary methods of converting the infrared radiation intovisible !ight. In an analogous way the same type of non-linear process can lead to higher harmonics aswell, thus expanding laser radiation out to much higher frequencies, tnat is into the ultraviolet.

4. SUI'ARY OF LASER PROPERTIES

(i) Although in reality an extended source, the LASER is effectively a point source.

(ii) Because the laser cavity can be caused to oscillate in one mode, the source is very monochromatic.In many materials the spectral width of the emitted radiation is more than six orders of magnitudenarrower than the Doppler breadth, so that the spectral brightness of the source is unequalled.

(iii)Laser licht is spatially conerent, similar to what we would expect from point source at infinity.

(iv) In most gas and liquid LA3ERS and in some con)figurations for solid stat. LASERS, the light thatis emitted is highly polarized.

(v) Laser light is highly collimated, with a divergence angle as small as the diffraction limit forthe wavelength emitted.

(vi) The brightness of the LASER is unmatched by any other source, particularly when one considers the

c-switched or pulsed high powered LASERS

S. TYPES OF LASERS

In the following short paragraphs I will try to describe the various types of LASERS, indicate thebasic science beh.nd their operation, and outline their basic parameters. The first five are solid state

LASERS, the ninth a liquid LASER, and the rest gaseous LASERS. In Table 1 I give a summary of the mostpowerful ones now in use today

1 0.

a. Ruby LASER4' - The heart of the solid state TableI

ruby laser composition is A1203 with 0.05% by weight HipmarlaserCr203 in it. Without the chromium, the crystal isknown as sapphire. In Fig.l we show schematically Lae Wave- M Peek Pule Laboraor

"the physical arrangement of the flash tube and laser Mium lengt ciency powe jut,.

crystals. Shown in Fig.j is the energy level diagramfor the chromium ion Cr which is the active species Nd: gis 1.4*Sm 0-2 ?xl•" I-5nSo S8hAe.Colkmbua.

in the ILASER. This is a 3-level system with two USA

laser lines, but only one, 694.3 nm, dc.-intes be- 4x10" sLe

cause of transition probability for this line is Liverfore. USA10'I tns KMS Fusion Inc.

greater than that of the 603 =m line. USA2xIt" 50ps Univ. Rochester

In all solid state lasers there is a serious USAproblem associated with the removal of heat from the 5x12' 2ns Lebodev.Moscow.15Salaser material, since large amounts of energy must U0 0 i- *x0 Ins Los ARamoa.USA

be introduced into the system in order to cause the Iodine I -Mo C l M-Iu4nichl.*L

inversion. In the case of ruby where the broad ab- Bathkn. Germasorption bands of chromium are near 400 :=0 in the Hydrogen 2.7m 110 10" 35 n Los Alemols a"blue, and 550 rm in the green, the high pressure fluoride (elec- Sandia. USA=ercury lamp can have as much as ten percent of its tim0

light absorbed in this region by the ruby. However, miche-

the efficiency of pumnirq. of ruby or any other solid mic a r

state LASER can be im:.. oved with the addition of Lo ndSnm <10-n 3x106 30 leq~d CANqo- Londont

other ions with suitable absorption bzx.ds incorpor- Xenon 173nm >2 4.100 Mns Los Alamos andated in the host latus. These ions then transfer Mlerell Labs.the excitation to the lasing ion, much as happens in Inc. USAthe case of the helium-neon laser which will be dis-cussed shortly.

The optical quality of the ruby is a critical factor in laser operation. Not only are scatteringcentres detrimental but so are all variations in optical path from one end to the other. The modestructure, divergence, and the pattern of the radiation generated are largely determined by optical pathvariations. One of the disturbing observations about the radiation emitted by the ruby rod with parallel

3.4

uniform end surfaces is that it does not emit coherent radiation uniformly over the surface. Small, verybright spots - hot spots - appear at the end faces which vary in size and intensity. These reflect thequality of the rod.

It should be noted that although the three-level ruby system is still among the most popuilr LASEP.S,it is perhaps one of the most inefficient because the terminal level is the ground state requiring thatslightly more than half of the atoms be in an excited st-ite for the system to work. By contrast, most ofthe solid state LASERS are four-leval eystems, which normally have efficiencies that are much greater thinthat for ruby.

b. Neodymium Crystal Lasersl - A neodymium LASER is characteristic of all the rare earth ion lasingsystems that have now been studied. As in the case of neodymium, all of the rare earth ion systems whic.ainclude both the spectral ions of R and R are important, and have been observed imbedded in a numberof host crystals and glass. By varying the ion and the host material on- can vary the wavelength of theLASER over an extensive range. A partial listing of these is given in reference. The list of rare earthatoms that have been used include all fifteen members of the I.ANT71IDE series ranging from La through Lu.

The big advantage of the four-level system ({Fi.4) over the three-level system iý that energy from aphoto flashlamp is absorbed in a very broad fourth level, transferred usually ron-zadiatively to .hc thirdlevel, followed by a laser transition between the third and ..acond level. The system finally comes toequiliDrium again in the ground level. The requirements with regard to inversior, are much less stringentthan in the three-level system.

The most frequently used host crystals for ne.dy-ium include CeW9- , SrW0, SrMoO•, Ca(1b03) andY3AISO1 2(YAG) with neodymium concentrations ot the order of 0.5 to 2%. Of these 7ttrium aluminum garnet(YAG) operating at room temperature at 1164.8 run is most co.mmonly used. In other crystals the neodymiumilines aFppear at wavelengths b, -ween 900 and 1350 nm. The substituti-on of the other rare earth ions leadsto a multiplicity of other levels in the same general region.

Although the neodymium crystal LASER can be run at room temperature it is much more effectively pumpedif the laser rod is at 77°K, sin-e at room temperature the terminal laser level is partially filled, where-as at l.quid nitrogen temperature it is not.

3 LEVEL SYSTEM 4 LEVEL SYSTEM

3 !>.

I t F 2E

Fgure 3. *ergy level diagram for F:gure 4. Schemi.tic energy level diagramthe three level ruby, syste. for the four level system.

c. ;eody.mium Glass Lasers - The neodymium crystal LASER is a useful tool for the research labo.atoryhowever where high power is neee-ed neodymium ions erbedded in class (barium crown glass being the mostfavorable medi!}) come cl'ise to the potential of the ruby LASEP. Neodymium glass laser rods that are twoto three m in length and three to four cm in dianeter are com.inly used. Such LASERS can -deliver more than5000 joules in a single pulse. As witn the ruby and crystal LASEkS, the glass LASER 4s excited by meansof a xenon flash tube. Furthermore, any of the rare earth ions may be embedded in glass giving laseraction over a wide range of wavelengths.

The -main advantages of glass as a laser host, are flexibility of sizze and shape of the rods and theexcellent optical quality. There is also a flexibility in some of the physical properties, in particularthe refractive index, which nay be varied from approximately 1.5 to 2.0 by selection of the glass. It ispossible to adjust the temierature coefficient of the index of refraction so as to produce therrillystable optical cavities. H",never, the major disadvantage of glass is the low thermal conductivity whichimposes limitations on continuous operation or high repetition rates.

Although tirmes shorter than 1 nsec can be obtained in the giant pulse laser systems by Q-s'.itching, itis possible by developing oscillations within the giant pulse to procuce peak pulses with half-widths ofthe order of p•icoseconds. 'the highest peak power achieved thus far by solid state LASERS are obtainedwithin the giant pulse mode in combination with respective ultra-short pulse techniques. These methodsare particularly important for specialized uses such as laser fusion.

d. Semi-Conductor LASLE , . - Of the solid state LASERS the semi-conductor LASERS are the most ef-ficient, and are by far the easiest to modulate, but they operate effectively only at very low temperatures.Unlike most other LASEPR. whe.e electrical energy is converted first into pkotons or into electrons that"Lombard the system, in semi-conductors it is possible to convert electrical energy directly into coherentlight. Such conversion takes place in the dinde injection !ASEPS in which excitation is the ir.rediateresult of work done by an imposed electric field on *-he charge carriers in tihe material.

S •. ..... . - , .:i l -- ' r ' l i I • • •_. -.... .... :1 - :......- - - = • • -: " •

3.5

A schematic energy level diagram for a PN junctiondiode is shown in Fig.5. Semi-conductor LASERS depend Zon radiative recombination of electrons ans holes of 0semi-conductors for their operation. Only certain semi- I= Iconductors, those such as GaAs with a direý,t gap be-tween conduction and valance bands are suitable. -- LE

Semi-conductor LASERS differ from other solid state STATESLASERS in most of the physical and geometric character-istics. They are two to three orders of magnitudesmaller in size than the typical crystal or gas LASER.The largest dimension of a cosmmon semi-conductor LASERis at most I mm. The relevant physical properties ofsemi-conductors and their variations with externalparameters such as pressure and temperature thus makethem good candidates ior tunability of energy. SA

Gallium arsenide occupies the same role among semi-

c-rnductors that ruby occupies among ionic crystals. It N JUNTION Pis the first and the most used semi-conductor laser - REGION REGION t REGIONmaterial. The band gap in GaAs varies with temperatureand in purity content and pressure. At 770 K the band >

* gap of the pure crystal is 1.51eV. At 300oK it is only Figure 5. Schematic level diagram for PN junctionaround 1.41eV. Electron-hole recombination is obtained laser excited with an electric field.from heavily doped GaAs diodes (77°K) with a spectral

distribution that has 4 peak between 840 and 850 rim. This corresponds to a photon energy between 1.46 andlo4•eV. Several hundred watts of peak power may be obtained from a GaAs diode in pulsed operation at 770 Kwhereas nnly 15 watts has been reported at room temperatures.

The light em.'tted from the diode is usually plane polarized but the polarization varies from one diodeto another. An effective emitting area is maybe as small as 2 Jim. As a result, the divergence of thebeam is about 100, much broader than beams radiated from ion crystal lasers. High power Jiode LASERS notonly emit in the near infrared region around 840 nm but also in the blue region 420 r=, twice the fre-quency of the infrared radialtion. The blue emission is the result of harmonic generation or frequencydoubling within the diode itself.

A large number of other iniection Laser systems have been developed with wavelengths which varythrough much of the spectral region. Furthermore, one of the advantages of using semi-conductors is thatthe wavelength can be shifted over a considerable range by alloying. Semi-conductor lacers can aisc beoptically pumped, or pumped by high-energy electron beam;, or by electric field breakdown within the system.

e. Organic Dye LASERS 1 3 - Although the organic chelate LASERS are built with rare earth ions and non-

organic niodeodymium-selenium oxychloride LASERS exIst, nost important of the liquid LASERS is the organicdye LASER. Such LASERS have emerged as the most versatile laser systems now available; both pulsed andcontinuous operation are possible. With phase or mode locking of the waves within the resonant cavitypulses as short as one picosecond have been obtained. The emiss;on from organic dyes is extremely broad-band. Consequently, as a research tool, its use is virtually unlimited, since with the proper choice oforganic dyes it is completely tunable over a range that rtns from the infrared through to the ultraviolet.In a recent article , a list of more than fifty organic dyes which can be used in the dye LASER have beengiven. Since that time many chemical companies have been active in developing new dyes.

4 WAVELENGTH (num)no Sao Mu 40M

So~t L.....o

S3

- T2 RHODAIINE 66

PuLaNG* soPHOTONS 7T.

LLArSER~

TRANSITINSAIs 0

Figure 6. Schematic level diagram for WAVE NUMBER(cm) xMO (V')the organic dye LASER. Figure 7. Absorption and emission curves for rhodamine 6G.

The operating principles are the same as any LASER (Fig.6). When optically pumped, dye molecules areraised to the lowest excited singlet state S1 either directly or via cascades from higher singlet stateswhich relax quickly to S1. Lasing involves the return to the ground state S by stimulated emission of aphoton. In practice the process is very complex. The light emission has competition from several otherprocesses, mainly the non-radiative conversion of S, to the state So and from inter-system crossing tothe triplet T manifold. In particular the accumulation of dye molecules in the triplet state Ti can bedetrimental to laser action if these triplet molecules absorb the light from the singlet system,"1 thusdiminishing amplification within the cavity. Shown in Fig.7 is the characteristic absorption of one of

3-6

the popular dyes, rhodamine 6G, as well ab its emisbion spectrur. If the laser cavity is not tuned to a

part'cular frequency the system will oscillate over a broad band. For example the characteristic colour

of ti'e rhodamine 6G emission is in the orange.

Typizal dyes are dissolvabli: in alcohol or water. In order tnat the efficiency remains high it is"necessary that they be cooled. As a result they are either circulated through the optical cavity or firedin a licqid jet stream through the optical cavity. For stability and r-,producibility the use of the jetis becoming more popular. Excitation of the organic dyes is accomplished by optical pumping using eithersolid state LASERS, nitrogen or argon discharge LASERS, or extremely fast flashlamps. Normally the gainachievable by using dye solutions is extremely high.

Initially dye LUSERS were found to operate only with very short pulses. However, a careful study ofthe quenching mechanisms have made it possible for the system to be run cw.

f. Helium-Neon and other Nobel gas LASERS - All of the gaseous Lasers which follow depend upon avariety of az.nmic and molecular collision pyocesses which include electron impact excitation, electron im-pact excitation through resonant processes, electron impact deexcitation (superelastic collisions) photo-excitation as in the solid state case, energy transfer from an excited atom or molecule to another,charge transiar between an ion and an atoi or molecule leading to excited products, etc. In the helium-neon LASER which is among the most used LASERS available today population inversion results from electronimpact excitation of the helium metastable states followod by energy transfer to upper radiative statesof the neon atom

e+ He -He + e' 21 He NoHe +Ne4He+Ne

Ne - Ne + hVl, hv 2 or hv3 .3

The schematic energy level diagram is shown in p

Fig.8. As indicated in the final equation, and as 20Fshown in the diagram, the helium-neon LASER operates W {in three distinct spectral zanges: in the red at 2S 2.1632.8 rnm, in the near infrared around 1150 nm andfurther in the infrared at 3390 nm.

He-N;e LASERS were first discovered in 1960 by Z IJavan et ails. Although there are three dominant 6 --I

" lies, as many as thirty neon transitions can becaused to oscillate, most under very special con-dtitions. As one can see in the diagram, the 632.8nm transition is in competition with the 3390 infra- lired transition. In order to cause the system tooscillate primarily in the visible it is necessaryto suppress the infrared line. This is done in a !number of simple and often very sophisticated ways 0in the commercial -ASERS. Because of its simplic- i'Sity and significant power in the visible the He-Ne Figure d. Level diagram for He showing first twoLASER is used primarily for instructional purposes metas'able states, which transfer energyand in the laboratory. Furthermore, it is the to the Ne le-els which subsequently lase.primary lase,' tool used for alignment and is asource of coherent radiatic. in holography. Its power output ranges from less than a milliwatt to powerswell in excess of a kwatt. Under normal circumstances the He-No LASER is run in a continuous mode, al-though it can be operated at higher power in a pulsed configuration.

Although the He-Nle LASER is the principal nobel gas LASER it should be noted that gas discnarges inhelium, neon, argon, krypton and xenon can produce atomic radiation that can be the basis of a LASER. Notonly are the pure gases used, but often it is found that mixtures produce enhancement of some of the laserlines. As a rule the output power of the nobel gas LASER is low. Under normal conditions they work inthe cw mode. In all cases -he laser configuration is the same as the helium-neon case. It becomes moredifficult to achieve stimulated emission at short wavelengths because of the required pumping power in-creases as the 3rd power of the frequency. The large emission band widths reduce the net qain of a givenpopulation inversion. These difficulties are further aggravated by the absence of effective sourcescapable of rapidly pumping the nc.•el gas to higher energy levels.

Until recently stimulated emissiin at ahorter wavelengths has been through the excitation of gases byhigh powered ns pulse discharges. N1ow hign energy-high current electron beam pumping has resulted in someof the shortest laser wavelengths observed to date approaching 110 nm. Recent progress in electron beampumping of vacuum UV LASERS is an outgrowth of work on -.ondensed nobel gases by Basov and his co-workers 1

7

at the Lebedev Physical Institute. In this paper the Russiana group demonstrated stimulation of 176.0 rmradiation in liquid xenon which resulted from the diatomic molecule of xenon making a transition to itsrepulsive ground state in a fashion similar to that shown in Fig.9. The final state of the xenon exciteddimer came about through a sequence of events such as:

+e(high energy) + Xe -" Xe + 2e

Xe + 2Xe - Xe2 + XeThis occurs in %10-10 sec at 10 atm.pressure. Ionization was then followed by three-body recombination

Xe2 + Xe + e - Xe + Xe

the final transition from the excited xenon dime- to the repulsive ground state represents the lasertransition which occurs in a time approtimately 10-12 sec.

g. Ion Lasersis - In principle, ion LASERS are similar to other gas discharge LASERS, however they

operate in the near infrared, the visible, and thenear ultraviolet. Ion LASERS operate with consider-able dissipatic.i of power but their peak energy out-put is usually orders of magnitude higher than those *of atoadc gas LASERS. They are not an efficient X4+ XeLASER, since in the discharge it is necessary to ex- +cite a level of an ion, thus reqrjir~ng considerableamount of energy, most of which ultimately ends upas heat. The output of the LASER is dependent uponthe square of the current. The first eltctronionizes the atom while the second excites it. Al- 1 -fflOthough many ions have been excited through lischarqesthe most popular ion LASlP is the 488 nm Ar . The W

argon ion LASER has become very popular, primarily Jfor therapeutic worx in ophthalmology. Beridesargon ions, neon, Krypton, xenon, oxygen, mercury, -

iodine, iron, chlorine, bromine, boron, carbon, h

sulphur, silicon, manganese, copper, zinc, germanium, 34+ X0arsenic, cadmium, indium, tin and lead ions Lave also

been used in ion LASERS as the active medium. The I . |cadmium ion LASER is now becoming very popular. a 4 6

It has beer suggested' that charge transfer INTJR IM DISTANCE (A)might be in effective method of producing ex-itationof radiation in the visible and L'V. In fact, for Figure 9. Level diagram for lowest two levelscadmium, zinc and tin, this has long been suspected of the high pressure Xe gas .aser.of one of the primary pumping mechanisms. Quite re-cently it has been demonstrated by a group at the University of Texas 2 0 that charge tiansfer of He? with

NZ leads subsequently to radiation of the nitrogen ion at 427 =m with an efficiency approaching 2%.

h. Molecular Lasers. 1 (Not including Chemical Lasers) - The most significant advances in laser tech-nology have come within the last five years in this area. It is probably fair to say that all molecularg&ses can be made to lase in ono, mode of oper.tion or another. As pointed out in Lecture 1, withxn amolecule there are combinations of electronic, vibrational and rotational tzansitions. Most of the mole-"-ular LASERS that have been marde operative have involved the vibrational-rotational transitions. However,a substantial number of transitions have been observed in th- infrared, the near infrared, --isible andultraviolet, associated with electronic transitions of a number of uiat•mic and triatomic systems. Themost useful electronic transitions thus far used have been in nitrogen, particularly associated with whatare known historically as the first positive, and second positive systems. In the first positive systemwhich involves the transition between B'I1 and A'Z+u electronic states. As much as 500 watt peak poweroutput has now been measured through the w~velength ranges 775 arn 758 nm. The transitions associatedwith the second positive system of U2 (C fu - ON ) lie ii, the near ultraviolet. More than 30 laser linesof this system have been observed between the various vibrational-rotational branches. Other groups oflines have been observed at 357.6 nm and 33;.' nm. Pulse powers in excess of 300 kW are routinely cbtai-ed.Other electronic transitions have been observed in the infrared. Besides nitrogen, H2 and D2 have beencaused to lase associated with an electronic transition.

The most significant work in the past five years has been associated w.th vibrational and rotationalexcitation of :'42 and CO 2 as well as mixtures of these gases with He and semetim".s minute impurities. Allof these systems have been caused to effectively lase with high powered output in the standard gas dis-charge laser tube. However, the biggest advance has occurred in several areas, partic. arly associated

22with the high pressure gas discharge systems . It is only these systems which 1 will consider, sincethey are the basis of many of present and future industrial and military uses of LASErS.

The high pressure systems include the TEA LASER (Transverse Excited Atmospheric LASER), the E beam andBlumlein excited LASERS and the electric discharge gas dynamic LASER. Before examining any of the tech-nical details let us consider the basic physics of the processes involved. In N2 the lowest vibrationallevel of the molecule in its ground electronic state is excited through the formation of giant Nz reson-ances in the vicinity between 2 and 3 ev. It is the resonance excitation that is primarily responsiblefor the large probability of forming N2 (v = 1). In mixtures of 112 and C02 , C02 in the first asymmetricvibrational mode is excited by resonant vibrational energy transfer from the v = 1 level of N2. This isdemonstrated in Fig.l0. The CO2 1060 nm transition then results between the first asvrmetric vibrationalmode and the symmetric vibrational mode while the terminal level is subsequently destroy.Pd through cascadeto the ground vibrational level. The vibrational levels of N2 are metastable and therefore represent areservoir of stored energy for selective and effective excitation of CO.

A pulsed transverse excited atmospheric LASER in CO2 was first reported by Beaulieu23 in 1970. Morerecently various methods of prelonization involving either electrons, heavy particles cr ultravioletradiation are now used in conjunction with the TEA type LASERS to obtain large volumes of gas discharge andthus more energy. Preionization results in large quantities of cnarged particles in the gas volume priorto the initiation of the discharge. These charges aid in the productioz. of a large volume glow dischargeof high spatial uniformity. In a recent embodiment the onset of the electrical discharge is controlled un-til the optimum de ree of ionization exists within the discharge volume. One such C02 system developed byRichardson, ei: al2 has given output pulse energies at 1060 an of 300 J in the multi-gigawatt range andwith an overall energy extraction efficiency apprcaching 10%.

The production of stable unif,)rm discharges at high pressure has also been accomplished using electronbeam (E-Beam) preionization. This technique involves the use of a hLgh energy (0.1 to 1.5 MeV) electronbeam to ionize the gas. An applied electric field accelerates the resulting charges and provides electric-al excitation of the laser molec The discharge is not ielf-sustaining without the electron beam. Ina recently developed system descri.,ed by Daughertys electron beam control discharge was produced in a 40litre C02-N2-He gas at 1 atm. The output is •2kJ and a pulse length n40Usec.

3-8

Increased operating pressure has O.led to greatly improved perform-nre bySYMWsk endn Ayshliincr2asing pulse energy peak power and vibroflo.ml:imum permissible repetition ýate. a1At very high pressures much greater | Valthan one atmosphere the disc ete vi-brational rotational lines broaden and 0.'

merge into a continuouz emission band.Such a LASER will now be tunable overa broad spectral ranje or mode lock lo 0o Niproduce picosecond (10-12 sec) pulses.Already, as reported by the Russians,C0 2-N2-He LASERS have been operated atpressures in excess of 50 atm. Cw W Csystems in excess of 25 kwitts havenow beirz develored in high pressure W"1 hiUflowing systems . One of the signif-icant advances in the study of thiedynamic aL .-r system has been the com-plete an..lysis and oredictability of Vj , V" Vthe system using the basic cross-sectaon available 2 . The depend- :2~Id lyn na levels

-nce of power output on the tempera-ture, pressure, gas flow, gas mixtureand impurity has been studied thorough- Figure 10. Level diagram for the first vibrational levelly, has led to a predictable increase f CO whc coe to ng vir o1).in efficiency. Of C02 which couple to N2 (v

The Blumlein pulse generator willnot be described here. It is sufficient to recognize that it can produce excitation currents of hundredsof kiloamperes at voltages of about 100 kv with a rise time near 2.5 ns. Because of the enormous poweravailable from this generator LASER action has been observed in many systems in the VUV region.

j. Chemical Lasers2 - Many exoth,'cmic chemical reactions lead to population inversion, primarily of

the vibrational and rotationAl states of the ground electronic state of the diatomic product. This processhas been studied by many, in particular J.C. Polanyi and his associates2 ' who proposed this mechanism fo6creating an inversion. Because chemical reaction energies can be very large compared with vibrationalenergy level spacings a reaction can produce molecules which are excited to very high vibrational levels.In fact a major part of the energy that is liberated in many chemical reactions leads not to kinetic energyof the fragment products but rather to internal nxcitation.

One of the most important chemical lasers involves the production of excited HF or DF molecules. Forexample in the H + F2 reacticn about 60% of the reaction energy appears in vibration so that the averagevibrational level of the HF molecules produced is v = 6. Because the spacings between the vibrationallevels of both the HF and DF molecules vary markedly from one vibrational level to another, a broad rangeof radiation wavelengths is emitted. In HF it ranges from 2600 to 3600 Am, while in DF 3500 to 4700 im.In addition each vibrational level has associated with it a large number of -otational sub-levels whichlead to a large total number of different laser wavelengths. 92 and F2 0'. not undergo a rapid directreaction. However in the presence of dissociated hydrogen or fluorine tney can react rapidly through theF+H2 and H+F2 elementary actiois, which together form a chain of reactions leading to excitation. In mostchemical LASERS studied thus far, it is necessary that free atoms be involved. Thes,: atoms can be formedin an electric discharge leading to cw operation or in a shock tube, or at high pressures in the TEA laserconfiguration, or in a flash photolysis apparatus where the cha.i of reactions is initiated by photo-dissociation.

6. APPLICATIONS OF LASER LIGHT 3 0

Over twenty-five years ago at the time of the invention of the transistor one could predict essentiallywhat has happened since that time. Because the transistor was an improved device for performing existingfJnctions, its evolution was comparatively straightforward. The LASER on the other hand produces lightthat is different in both quality and intensity from light generated from any other source. Consequentlysome of the more obvious uses of LASERS in existing systems, such as conventional interferometry will turnout to be less important than thb- develnpment of new systems that take advantage of the unique character-istics of laser light. Perhaps the best example of this has been the rapid development of the new fieldof interferonetry known as holography, that is, the storage of three dimensional information in an inter-ferogram. Though not a new idea the potentials of holography were realized only after the invention ofthe LASER.

Because of the highly specialized properties of the LASER which were discussed at length in comparisonwith other sources at the end of Lecture 1, properties such as brightness, spectral brightness, monochrom-aticity, spatial coherence, the fact that laser light effectively comes fror a point source make possibleapplications and developments which today have not even been considered. Anj list of applications pre-pared unquestionably will be out-of-date within a few years. The important Ioint to remember is thatlaser light has now become an integral part of man's everyday experience, in the home, in the office, inthe shopping centres, in manufacturing, comunication and power production, in our military arsenal, andprimarily in the research laboratory. The brightness of the source and the fact that it can deliverenormous power make it a hazard for man, particularly the most sensitive part of man, his eye. As we con-sieer the various applications, try to keep in mind how in each this hazard exists and can be minimized.In subsequent lectures many aspects of this problem will be discussed.

a. ••SERS in Metrology - Laser technology is important in the determination and maintenance of stand-ards. However, the performance of He-Ne LASERS stabilized by .4eturated absorption in methane at 3390 nm

• •3-9 2-1i

and iodine at 633 nm is s ich that they are being considered as fr( jency standards. These laser systemshave shown a frequency stability comparable with the cesium-beam .zequenci standard now acceptec. and areproducibility much better than the krypton-lamp length standard now universally used. Such referencesystems are now cormercially available. Based upon the accepted trequency and length standards, thevelocity of light is now fixed at 299,792,458±4 .n/szc. Even allowing for the improvement with LASERS, thevalue is not expected to change.

b. Communication and Information Storage - LASERS will no doubt have their largest impact on the totalhuman experience through the areas of communications, information storage at.d retrieva". At optical fre-quencies the band width is such that all the information presently transmitted through all telephonecircuits, all radio, television and radar systems in principle, can be carried on on.- laser beam, providedthe .ogic were available to code and decode the signals in orderly fashion. Besides the large informationcontent that can be included in a single optical beam, one has the definite advantage that in opticalcomunication systems the normal electromagnetic interference is not a problem. Fr=r the point of view ofmaintai.aing cor.munications during times when there are natural atmospheric interferences, such as from anelectr:cal storm, or durini tines when enormous interference is generated from an atomic blast, theoptical corziunication system is not affected.

information can now be carried directly on lcser beams through the atmosphere for short distances.However, dust, temperature, fog, to a large extent, interfere with the continuous use of such systems overlong distances. In outer space, the situation is different, and already LASERS are being used for com--un-cations between satellites. To eliminate tb..s problem laser light is now being pired through smalloptical fibres, and in tine it is clear that most communication at telephone, radio and television fre-quencies will be carried in small fibre bundles.

.he developent of se-i-condu.tor LASER3 with active areas small enough to readily match the opticalwaveguides as part of integrated o.tizal systes now make possible the in-line =plification of the signal

_ibre-opntc tansmission lines. Sesid.s amplification of the signal, the semi-conductor LASER affords

a high efficienc- method of coupling the signal onto the optical frequency carrier w~'ve.

If the amount of --nformation stored in scientific and technical journals, n,. corporate and govern-ment files cont'nues to grow at the rate that it is growing today, the weight of tne material might well

approach that of the earth by the early 21st century. In restonse to the exponential growth, we have al-ready begun to store information on microfilm and n co.-nuters. 'owever, even with these sto.gap measures

- .s clear that we will lose the information storage and retrieval battle unless new techniques aredeveloped. Such technicues are nresently under investigation and in l:mited operatioz. today.

The heart of the storage system is the holocram. The information containing interference pattern isfrozen with-n the photogranhic emulsio:, or sore special crystals. Cr 2,stals sucn as lithium nicbateL- 120 3 have the nrorertv that =any hundreds of thousands of holographic interference patterns cin De storedin one very small single crystal, and can be ea!ted or rc-overed at will. To look at a hoiogra-, one seesa hodgepodge of specks, blobs, and whirls, whý.ch rezrosent the frozen :nterference pattern. By trans-mission of laser light through the hologram. or the reflection of it from the holographic crvsta, one canreconstruct the image not only in twc dimen- ions, hut in three dim:ensions. iWritten material can be storedas a ,oeries of holograms in page sec.ence. One holograrhic cube can be used to store a million pages ofmaterial. The information stored in the ho:ogra'hic library can then be transmitted to universities,cnF•anies, even przvate homes from: the central library through l-ght pipes to a television receivez. Notonly can :nformat:on he stored .n threi_- drmensions, and in bulk, but it can be stored in multiple cjlour.in the last few y.-ears the application of holocraphic techniques, coupled with digital computing techniques,and television moni;torina, a field now called optical computing, has contributed much to our ,nderstandingof the problem of storage and retr:eval of data. Further-more, it has made possible the active and con-tinuing proces:sng of infcrm.ation on a t:..-- scale toat has never before been imagined.

c. Product=on and transmission of power - .-he use of -ASERS in Powwer production is -Coupled intimatelyw-,th nuclear technolgy. In thenuclear f:ision :roc-ess a large p.,-rcentage of the cost is associated witha serarat:on of the uranium-. ioso,-oes. it has nwe been demonstrate': and will soon be the basis of com-mercial processes that hiqh powered LASER can be used tc excite and subsequently ionize uranium atomsassociated with a nart,:ula r isoto-.'. As one scans the technical literature today on LASERS, a largepercentage of it is associated w-th isotor,-ne se:.aration.

-or t:•e Canadian reactor, unike the Am-er:can syste., the uranium that is used is unenriched. Nearly40g cf the total cost of reactor produced .ower is associated with the production and handling of heavywater, the roderator used in the Canada ZAND reactors. .ormally. the heav.y isotope of hydrocen, deuterium,repr-sents approx:mately one p..rt and !07 of the hydrogen atoms available in nature. Of the total cost ofproducing neavt-y water, 70ý is associated with the first factor of 10 enrichment. Once again, LASERS arebeing used in an attemn_ to attach thoe zc le:ul,-s -tat are deuteri-m containing, in an attempt to especial-ly separate them from the larme bulk of material.

Probably the greatest call for high .:o-wr laser technollogy- is in the area of laser fuqion. In orderfor fusion to occur, it is necessary that either nigh •owered gas, or glass LASERS, be developed that willdeliver terawatts (10" watts, in times app-)roaching picoseconds, that is 10 1 •seconds. Such giant systemsare nearing completion today. High powered, high pressure gas L-ASP.•S are now working with efficienciesthat an,proach 50_. it therefore becomes reasonable to imagine that laser systems can be considered for thetransmission of power over long distances, and into awkward places. Unfortunately in the infrared, waterrapour is a strong absorber, thus making it difficult to transmit power at these frequencies.

d. Lasers in the co.-unity - Besides the obvious application of laser technology to communicationsthere are a nunlber of other uses that are now being developed. Automated checkout for the supermarkets

:,romises to he a .Iti-million oollar market for laser systems. At present the helium-neon LASFR has as-suned a place alongside the integrated circuit, and the semi-conductor memory as a reliable electroniccomponent in these systems.

S..

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Because of their high efficiency and brightness, LASERS are playing an increasingly important rol, indisplay systems. Furthermore, the possibility of eventually using injector LASERS for light bulbs is

certainly real. For the moment, the LASERS tha have been developed do not operate in the blue region ofthe spectrum. The efficiency of the present li.,:,t bulb is approximately 10%, and their life is short. Ablue diode LASER, such as SiC may be able to operate without being cooied with an efficiency approaching25%. The cohercnt monochromatic radiation that would be produced could be converted to hetezochromaticlight by surroumding the LASER with the proper type of phosphor which would efficiently absorb the laserlight, and reem. t it over a broad band of frequencies. Such a system would be extremely simple, and long-lived. Before the application of infrared laser light to the cleaning of works of art, such as statues,and national monuments, the process has required many man years of painstaking labour to scrub the dirtfrom the surface with sand. N;ow with the aid of the high powered infrared LASER these objects dart canliterally be scr-9bed with light. The light is preferentially absorbed by the soiled surface and thepreferential heating of the dirt causes it to be boiled from the object. The same principle has beenused with the laser eraser which is capable of vaporizing ink from paper without appreciably heating thepaper. Muse-s have now included holographic techniques in their arsenal of weapons used in determiningauthenticity of works of art.

e. LASERS applied to pure and applied science - Lasers find their greatest application in scientificlaboratories. The most obvious application of course is as a tunable light source reaching from the sub-millimeter range in the far infrared through now to the vacuum ultraviolet. The obvious primary use is ofthe tunable light source in conjunction with the standard spectroscopies. The spectral brightness of manylaser sources makes them ideal for studying properties of atoms and molecules which otherwise could not bestudied. With the aid 6f the LASER, investigation of non-linear optical phenomena has grown rapidly.Prior to the advent of the LASER in 1960, the electric field strength associated with ccmnly occurringintense light sources might be in the vicinity of 1000/m volts/,. With the advent of the LASER, electric

12field strengths produced by LASERS a,. -ow well in excess of teravolts/m (10 volts/n).

In much more modest fields multiphoton processes begin to occur within the material which lead tooptical harmonic generation. The crystal potassium dihydroqsLn phosphate is one of the materials oftenused for this purpose. Often the efficiency for producing second harzmonic frequency generation maybe Jnexcess of 20%, although typical conversions are between 5 and 10%.

As laser light interacts with gases, liquids a-d transparent solids, it is scattered both elasticallyor inelastically. Elastic scattering is called Raleigh scattering, while the inelastic scattering oflight is called Raman scattering. inelastically scattered light will contain lines corresponding toenergy loss in exciting various rotational, vibrational and electronic states of the medium. If the lightis intense enough it will also contain a series of lines correspondirg to the addition of vibrational,rotational, electronic energy to the light of the LASER. This then becomes another very powerful toolfor studying the internal structure cf materials.

Essentially, Brillouin scattering in solids and liquids is the same process as Raman scattering.However replacing the vibrational rotational, electronic excitation is the Aotion of an acoustic wavewithin the material. The frequency of these acoustic waves car. be added and subtracted from that of thelaser light thus giving a rich spectrum refl.!cting their magnitude within the material.

Within t1e labcratory LASERS are often used as intense sources of radiation for pulse radiolysis. thatis tle time study of a system after enerc-, has been rapidly introduced into it. Furthermore, the LASERis an excellent source of radiation for studying the interaction of nwn-ionizing radiation with livingsystems. For e.anple in my laboratory, our primary in~terest is in studying laser radiation damage withinthe retina. We al•o use laser light to assist with detailed studies of basic mechanisms in colour vision.

The laser is now important in cellular microscopy. The effects of laser radiation upon the cell havebeen studied by a numb.r of laboratories. The laser niuroscope also provides another instrument for micro-surgery of tissue cells and orgenelles, Laser radiation has now been used to monitor reactions in livingsystems inv',ving brain cells, DNA, and RHA molecules. Because of the monochromaticity of the laser lightand the small divergence of the beam, experiments can now be carried out down to sizes which approach one-half m~cron.

f. Industrial applications of LASERS - LASER technology is finding its wao, into virtually everyaspect of industrial processing. The most dramatic application of LASERS of course is in industrial metalwelding, dril,.ing and cutting, ceramic machining and drilling, fabrication of high precision resistors, ofprinted circuitry, manufacturing standards control, package labelling, and so on. Let us consider a fewmore detailed examples.

This past year some of the underbodies for the Ford Montego and Torino are being welded with a 6 kwbeam from a carbon dioxide LASER which was developed in the laboratories of United A.xcraft. Similarlythese high powered COa systems are being developed for ship welding, thus cutting by ten the aunt oftime necessary for fabricating zhip hulls. As with most laser systeis used in industry, the weldingsystem is invariably computer controlled. Laser beam welders are also important in the manufacture ofautomobile batteries (lead acid batteries) And in heat treating and surface hardening of such importantparts as camshafts and valve seats. There appear to be definis advantages in using the LASER for heattreating since the rapid process leads to the minimum amount of part distortion.

As in the case of heavy manufacturing, the LASER is of importance in the chemical industry. Asmentioned above, it is now effective in isotope separation of both uranium for fission reactors,potentially for producing heavy water as a moderator in the heavy water cooled reactors. Over the nextfew years its full potential will no doubt he 3eveloped.

g. Applications of LASERS to medicine - The largest single use of LASERS in medicine is in thera-peutic photocoagulation of ocular tissue. Up until the development of LASERS the greatest advancementhas been the xenon Arc lamp; however, with LASERS one can now control the power, the spot size upon theretina, the irradiation time with the tunability of colour to match the absorption spectrum of the

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material under irradiation.

Photocoagulation has now been extensively used in treating a number of diseases of the macula. Forexample, the majority of patienLs treated for serious central retinopathy have shown an improvement invisual acuity within three weeks. However, diabetic retinopathy is rapidly becoming a chief causc; oiblindness. It is now estimated that approximately 19% of the blindness in the U.S.A. is caused by suchretinal changes. Coagulation of the retina is one of the major approaches to the control of this disease.Although the ruby LASER, which emits at 694 nm in the red, has been used, it has not been particularlysuccessful. Instead, either the argon ion LASER which emits at 488 and 514 nm or the frequency-doubled-neodymium doped YAG crystal which emits at 530 =m have more successfully been used. The relatively highabsorption of the green wavelength by reduced or oxygenated hemoglobin makes these latter two lasers veryattractive in the treatment of retinal vascular anomolies. Treatment of glaucoma, by poking a small holein the iris with the LASER, has thus far been carried out in Russian laboratories.

In recent years, the LASER has become a surgical tool. Both the infrared CO2 (10.600 nm) and a greenargon ion LASER (488 and 514 nm) have been effectively used as these radiations interact quite dramatic-ally with tissue. The red ruby and He-Ne light are not appreciably absorbed by tissue, blood or water andconsequently are of little use. The advantage of laser surgery is seen in the bloodless cut since vesselsscar immediately. Attempts now are being made to use laser surgery in awkward places such as in the skullfor the removal of cysts.

6ecause of "-he high power density and the monochromaticity which sets the defraction limit of the spot'ssize, the LASER is an excellent tool for microsurgery. Once again the choice of the critical wavelengthis important since one is able to irradiate part of the subsystem of the cell with that frequency of lightwhich is be:t absorbed by it.

The LASER is also being considered as a tool in dentistry. Thus far it has not readily been acceptedbut in the fu-ure it may be important in the treatment of special diseases and for mechanical constructionin awkward places.

LASERS have also found extensive use in dermatology, particularly in those areas involving cosmeticchanges such as the removal of tatoos, birthmarks, and growths. The early enthusiasm that developedaround laser surgery associated with cancers has now lessened because it has been observed in many in-stances that treatment by the LASER has cpused the diminishing of the original cancerous growth but hasalso caused it to spread to other areas.

h. Mining and Geological Applications of Lasers - One of the most cocon uses of LASERS now is insurveying. However, the -onochromatic properties and its high spatial coherence have made it a superbtool for interferometric r-easirements of small earth crust movements. Extensive study has gone into thedistortion of the earth's crust with the motions of tides and of earthquakes, and with the aid of theLASER, scientists throughout the world are n=w able to make predicitions as to when and where major earth-quakes will occur.

The extreme power of the YAG. CO2 gas LASER and some chemical LASEPS make them excellent candidatesfor drilling and mining. Already LASERS are In the field in these areas.

LASER light was bounced from the moon. As a result, scientists have been able to determine veryaccurately the shape of the earth.

Laser radar or LIDAR is now playing a very important role in determining and monitoring pollutants inthe lower atmosphere and the LASER is now playing a particularly important role in map-making.

j. Military applications of LASERS - Virtually every laser application thus far discussed finds a usewithin the military. Conversely, the hundreds of millions of dollars spent on laser-related research anddevelopment supported by military estab.ishsents not only finds application there, but has quickly foundits way back into the community.

Information storage, processing and communications are of primary importance to the military. In-tegrated optic systems, which allow for coupling of the compucers through optical fibres without electro-magnetic interference are now commonly used in military systems. The use of holographic storage ofinformation and the holographic techniques in -ap-making are now under consideration. The use of opticalcom•unicators between aircraft and between line posts are now under design. Some are presently in thefield, as are .aser range-finders and guidance systems.

The power associated with modern LASERS is sufficient for anti-personal weaponry. However, the mainthrust will be in developing LASERS that can be used to ignite thermonuclear devices, and to detonate suchdevices in MERV war heads.

Although not strictly a military application, one of the "far-out" applications for the future will bethe use of LASERS for space ship launching and propulsion in space. Such schemes are presently under studyat NASA and have been proposed by such leading experts as Dr. Arthur Kantrowitz, Chairman of AVCO ResearchLabc "atory. The magnitude of the LASERS necessary for such a scheme is mind-boggling; however Dr. EdwardTeller, his teacher, was asked to comment upon the Kentrowitz proposal predicted: "It will happen beforelaser fusior. will make a contribution in a practical sense. I am interested in... how soon the fusionenergy we want to squeeze out of these microexplosions will rea-ly give economic power. And I believ.propulsion of manned satellites will occur before that occurs."

S. . . . . . .'- -

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1. N.G. Basov and A.M. Prokhorov, Zh. Ek-perim i Theor. Fiz 27. 431 (1954).

2. J.P. Gordon, H.J. Zeigez and C.H. Townes, Phys. Rev. 95, 282 (1954).

3. A.L. Schawlow and C.H. Townes, Phys. Rev. 112, 1940 (1958).4. G. Chapline and L. Wood, Physics Today 28, No. 6 40 (1975)

G.C. Baldwin and R.V. Khokhlov, Physics Today 28, No. 2 (1975).

5. F.J. McClung and R.W. Hellwartho J.Appl. Phys. 33. 828 (1962).

6. R.J. Collins and P.P. Kisliuk, J.Appl. Phys. 33, 2009 (1962).

7. e.g., B.H. So'fer, J1. Appl. Phys. 3-5, ?51 (1964).

8. e.g. P. Lalltmand and N. Bloembergen, Phys. Rev. Lett. 15, 1010 (1965).

9. R.S. Adhav and A.D. Vlassopoulos, Laser Focus 10. 47 (1974).

10. D.J. Bradley. Endeavour 122, 90 (1975).

11. B.A. Lengyel, Lasers. (Wiley-interscience: N.Y., 1971).

12. B. Lax, IEEE Spectrum, July 1965, p. 65.

13. e.g.. P.P. Sorokin. J.R. Lankard. V.L. Moruzzi and E.C. Haond, J.Chem. Phys. 48, 4726 (1968).

14. J.T. Warden and L. Gough. Appl. Phys. Lett. 19, 345 (1971).

15. L. Allen and D.G.C. Jones, Adv. in Phys. 14, 479 (1965).

16. A. Javan, W.R. Bennett and D.R. Herriott, Phys. Rev. Lett. 6, 106 (1961).

17. N.G. Basov, V.A. Danilychev. Yu.M. Popov and D.D. Khodk-.rvich, JETP Lett. 12. 329 (1970).

16. W.B. Bridges. Appl. Phys. Lett. 4. 128 (1964). err 5. 39 (1964?.

19. J.Wm. McGowan and R.F. Stebbings, Appl. Opt. (Chem.Lasers Sup.21 . 68 (1965)

20. C.B. Collins, A.J. Cunningham and A.J. Stockton, Appl. Phys. Lett. 25. 6 (1974).

21. O.R. Wood, Proc. IEEE 62. 355 (1974).

22. A.J. Denaria, Proc. IEEE 61, 731 (1973).

23. A.J. Beaulieu, Appl. Phys. Lett. 16, 504 (1970).

24. M.C. Richardson, A.J. Alcock. K. Leopold and P. Burtyn. J.Quant. Electron QE-9, 236 (1973).

25. J.D. Daugherty. VII Quantum Electronic Conf., Montreal, 1972.

26. C.O. Brown aýd J.D. Davis, Appl. Phys. Lett. 21. 480 (1972).

27. e.g., W.J. Wiegano and K.L. Nighan. Appl. Phys. Lett. 22, 583 (1973).

28. A.N. Chester, Energia Nucleare 21, 23 (1972).

29. J.C. Polanyi, Appi. Opt. (Chem. Lasers Suppl.21 109 (1965).

30. Modern Applications drawn from vol. 9, 10, and 11 Laser Focus.

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IHNSTRUNMENTTION AMD MEASUREMENT OF LASER RADIATION

DAVID H. SLINELASER MICROWAVE DIVISION

US ARMY ENVIROMEAL HYGIENE AGENCYAWERDEEN PROVING GROUND, MD 21010

In the past decade, many new laser instruments and measurement techniques have evolved. The measurementsof primary interest in the evaluation of lasec hazards are: output energy or power, pulse duration, beam"profile and divergence, and pulse repetition frequency (PRF)o The most useful types of detectors and beamprofile methods will be discussed. Short-cut check tests will also be given.

1.INTRODUCTION. in any discussion on the measurement of laser radiation for the purpose of evaluating

S~health hazards, I feel it important to explain first the necessity of measurement. Industrial and

environmental health speczalists and health physicists rely heavily upon measurement in their analysis ofenvironmental hazards. it is not surprising, then, that one of the first questions asked uponencountering a potential laser hazard is: How do I measure laser radiation, and what instrument do I use?A decade ago I asked this question. I was soon to learn that the subject was very complex. Unlike manyhazards we encounter, a laser beam Is almost always hazardous and indeed, its hazard far exceeds amarginal condition. The output irradiance of most military lasers exceeds exposure limits by orders ofmagnitude - typically a factor of 10,000 or even a million times. One then realizes that the correctquestion may then be: Why should I measure this laser beam? Clearly, no one should place his eye, oreven in some instances his skin, into that beam. Well perhaps we should measure reflections. I was svoonto learn that trying to measure reflections was very frustrating. The slightest change of a reflectingsurface, the insertion of a different surface into the bean, a mode change in the laser beam, or amy of amyriad of other changes in the environment greatly affecte. my measurements. it soon became clear to methat routine measurements to monitor either an area or an individual by instrumentation was a hopelesstask. It tas necessary to develop an approach of analyzing the potential hazards of a laser based uponthe laser's output parameters. The laser beam's hazard can be compared closest. I think, to an exposed Ihigh-voltage conductor - a highly localized hazard. Unless you touch the conductor nothing happens. The

f beas is unlike an area hazard presented by a contaminated atmosphere, unless a hazardous diffusereflection or associated hazard mxists. We now conclude that any measurements of laser radiation must -V

reflect the need for determining all potential future exposure conditions. Our standards in the USA nowrequire measurements only of the laser output as a general rule for the purpose of determining the laserclassifications. In the military environment it is often necessary to measure bea& characteristicsdownrange. Routine monitoring is seldom considered necessary, and measurements are performed at one tineby or for the laser developer.

2. L.ASR PARAME~TERS TO M4EASURE.

a. One can calculate the irradiance (E) in watts-per-unit-area or radiant exposure (H) injoules-per-unit-ar"a at any distance from a laser. To do this the output power (0) or energy (0), theinitial beam diameter (a) and bean divergence (6) must be determined. The relation is:

S_ e_ _ 1.27 e1'

_____ 1.27 QeldtH -

/a + To (a + ro

Where r is the distance from the iarer. (i)

One can use a calorimeter or other types of energy or power meters to measure the output energy or power.The measurement of output beam diameter or divergence can be more difficult. The procedure my associatesand I prefer is the use of calibrated apertures with the aforementioned meter. Figure 1 shows the profileof a perfect Gaussian beam profile which is characteristic of a single-node laser. Figure 2 shows therelative power entering an aperture relative to the beam diameter. We specify the bean diameter at l/e ofpeak-irradianre-points, since the total beam power 0 divided by the area of a circular beam defined atthese points results in the irradiance of the beam at the peak of the profile in Figure 1. It isconventional for many laser manufacturers to specify the beam diameter at 1/e

2 of peak-irradiance points.

Using this latter definition one would calculate only the average bean irradiance, which is insufficient

for safety purposes. Beam divergence is simply the ratio of the change in diameter D of the laser beamwith the change in distance r from tne laser. If we measure the beam diamter at two locations by usingour aperture technique we have measured the divergz-,ce.

b. Regrettably the beam profile is not always single-mode. This is particularly a problem with rubylasers. Figure 3 shows several profiles of an emergent beam from a ruby laser. Measuring beam diameterand divergence of this laser can be quite a problem. With the single-node laser, one can calculate beanirradiance, or how much laser power is entering - 7-ma pupil or a 1--am aperture based on two or thre-measurements. Since the ruby laser profile changes rapidly there is no such simplified method to performcalculations based upon some "effective bezA diameter.

U5., of trademarked names does not imply endorsetwit by the US Army

4-2

W

Ž4WI-ac4 E

1.0

/

-0.5

J

2

CENTEROF

BEAM I

Figure 1. Irradiance Profile of a Single-Mode, Gaussian Laser Beam

98

u• 95 -

& . 60 -2

_ 40 -LL.0o

0 4

0 2 010o-

-.IU 50aJ

2 20 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

RELATIVE BEAM DIAMETERFigure 2. Percentage of the Total Laser Beam Power Which Passes Throuqh a Circular Aperture; Gaussian Beam.

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c. From the standpoint of hazard analysis it is necessary to know the maximal output radiant exposureof a pulsed laser to determine if a diffuse reflection hazard exists. The most effective technique wehave devised for this purpose is the use of thermally or photochemically reacting surfaces, orphotography. In other cases where the beam irradiance is insufficient to cause a surface charge in thespecial beam-profil, paper, then a radiometric instrument must be used which nas a sufficiently smallaperture. Downrange the atmosphere has perturbed the bear. profile considerably as shown in Figure 4.Measurements at this distance with an instrument having an aperture of 7-nm (related to the eye's pupil)or 1-mr in diameter are then required. Now that we have discussed the relevant measurements, we canconsider the instrumentation that is available.

3. TYPES OF RADIOMETRIC INSTMMMT. Radiometric instruments of interest to this dincussion generallyconsist of a detector which produces a voltage, a current, a resistance change, or a charqe which ismeasured by a sensitive electronic meter. We will not worry about rthe readout meter of the instrumentsince that seldom determines the selection of the instrument. I prefer taut-band meters because theyindicate fluctuations in the radiometric quantity. Others prefer digital meters. But I will caution you:

Smost digital readouts are difficult to read in daylight illumination. The detector is the primarydetermining factor in selecting an instrument. Each type of detector, be it a quantum detector(ph•tovoltaic, photoconductive, or photoemissive) or a thermal device, has certain characteristics which

may be an advantage or a disadvantage for measuring a certain level of optical radiation in a certainwavelength range. No one type of detector can serve for measuring all types of laser radiation. A vervsensitive detector can be destroyed by a high power laser bean. A detector sensitive to visible light maynot respond to infrared, which is a disadvantage if you wish to measure an infzared laser, but anadvantage if you wish to measure a visible laser and do a.ýt wish the detector to respond to extraneousthermal sources. Table I provides the approximate ranges of xzrLaia::ce and other radiometric parameters

* of interest to us for serveral wavelength ranges.

a. Thermal Detectors.

(1) Thermopiles and disc calorimeters, are characterized by a relatively flat response relative towavelength. The spectral response is dictated by the black absorber, such as gold black, parson's black,or Nextal C, which normally coats a metal surface. The temperature rise in t:•e metal is then convertedinto an electrical voltage or current by one of several effects. Because of the thermal mass of this"metal, the time required to heat or cool the target, limits the response time of the instrument. Inrecent years response times have been shortened by using thin-film techniques. Instead of a copper discor other large ",etal surface which is useful for measuring radiant powers of the order of I my to 100 W, athin film of metal which has been vacuum deposited on a nonconducting substrate is used to form athermopile. Lower powers must be measured, typically 0.01 to 100 mW, but the response tire can be lessthais r second instead of seconds.

(2) Response times of calorimeters and thermopiles may still be too great when one must measure ashort-pulse laser. Recently a class of detectors which exploit the pyroelectric effect have beenintroduced. Rather than responding to a final temperature elevation in a metal, pyroelectric detectorsactually measure the rate of temperature change in a crystalline material. Response times of the order ofnanoseconds are currently achieved in commercially available detectors. A CW pyroelectric detector isachieved by chopping the input beam so that the temperature rise in the crystal is always changing. Aword of caution is appropriate with these CW pyroelectric power meters: Do not try to measure the average

* power or irradiance of a repetitively pulsed laser, since the laser pulses may or may not pass through thechopper and since the detector is only calibrated with a CW source.

(3) Thermal detectors find their greatest application in measurement of lasers which operate in theinfrared region, where other detectors do not respond, or where other types require cryogenic cooling.

-•For a single instrument to measure laser power between 10 nW and 100 W, disc calorimeters are consideredvery good for all optical wavelengths. Through the use of appropriate entrance apertures, the meter canbe calibrated to measure irradiance. In many instances radiant energy output of a pulsed laser can bemeasured using a disc calorimeter if the beam radiant exposure is below tie damage threshold of theabsorbing black which may typically be of the order of 1 j-cm-2 or less. For higher energy pulsed lasers,a ballistic thermopile has often been useful. The disc calorimeter and the ballistic thermopile are bothmore suitable for the laboratory than for the field, since several seconds or even minutes are requiredfor the detector to cool between measurements of a pulsed laser or for stabilization in a CW measurement.Additionally one is always plagued by a changing ambient temperature resulting from drafts in themeasuring environment.

b. Quantum Detectors. These detectors are by far the most sensitive detectors of optical radiationin the 200 nm - 1,100 me spectral region. The spectral sensitivity of photoemissive detectors dependsupon the photocathode material used in vacuum photodiodes or photomultiplier tubes.or in the intrinsiccharacteristics of silicon as shown in Figure 5. Silicon is employed, in solid-state photodiodes, whichmay operate as either photoconductive or photovoltaic detectors. The type of detector chosen normallydepends on what wavelengths you wish to measure and what wavelengths you wish to exclude. Response tinesof the order of a nanosecond are possible with quantum detectors. The one instrument that my associatesand I have found the most useful for hazard analysis of all types of ultraviolet, visible and nearinfrared lasers utilizes a bi-planar vacuum photodiode detector. With the appropriate selection of inputoptics and apertures it is possible to measure radiance, integrated radiance, radiant exposuree, radiantpower or energy, and irradiance. The disadvantage of this type of instrument is that it can become quiteexpensive - of the order of $5,000 or more - to have all these features, with sufficient sensitivity.Because of the strong spectral dependence, these instruments are normally not direct reading and the meterreading must be multiplied by one or several calibration factors. Because the 0.4 um - 1.4 um spectralregion is the retinal hazard region, the levels of radiation required to be measured can be quite small,and the problems of filtering out ambient light and near-infrared radiation can be severe. The use offilters with these types of detecters normally presents great difficulties in the field, since narrow-bandfilters have a strong dependence of transmission varying with the angle of incidence of laser radiation.

44

Figure 3. Emergent Beam 7-ofiles for a Pulsed Ruby Lasei. Top-Central Pattern is the closest to single-mode operation.

TABLE I

APPROXIVME RADOE c RAGES OF INTEREST FOR HAZARD ANALYSIS

Spectral Region Irradiance Radiant Exposure Radiance Integrated Radiance

(CIE Band Designation) (W-cz- 2 ) (J-cm-2 ) (W-cm-2sr- (J-c 2 sr-1 )

Actinic Ultraviolet,UV-B & UV-C. 10-7 to 10-2 10-4 to 10-1 N/A N/A200 nmm- 315am

Near Ultraviolet, 0 tUV-A 10 to 10 1 to 10 N/A N/A320- 390 nm

Visible 10-7 to 10-2 10-7 to 10-2 10-1 to 103 10-3 to 10400 nm - 760 nm

Metavisible or NearInfrared. IR - A 10-6 to 10-1 10-6 to 10-2 10-1 to 103 10-3 to 102

760 - 1400 r=

Fax-Infrared,IR-B & IR-C 0.Ul to 1.0 0.001 to 10 N/A N/A1400 nm- l

Figure 4. Profile cf a q-switchedRuby Laser Beam at aRange of 1 km from theLaser. Only Part of theBeam is Shown to Illus-trate Turbulence Effect.

4-5

SI s I i I il I I I

l

lipp

UU

gU

1; T

• ~ ~ ~ ~ C I$* ! ! | L.

9)

200 400 0oo Soo 1000 1o00

WAVELENGTH (isn)

Figure 5. Responsivities for Variu.. Detector Types. SP: Schottky photodiode; IR: S-1 photocathode(Ag-O-Cs); V: S-20 photocathcde (Na-K-Cs-Sb); UV: S-S photocathode (Cs-Sb); T: thermopile.The curves shown are for bi-planar vacuum photodiode detectors, thermpiles and photodiodesused in standard field survey equipment in use at the US Army Environmental Hygiene Agency.

4T.---

4-o

Besides spectral filtering, geometrical filtering, such as the use uf a detector hood, may be used toreduce errors introduced by ambient light. In the final analysis, ioe technique is as effective asperforming the measurements at night. This is what we normally do.

c. Hazard Evaluation Heter.

(1) At present no radiometric instruments are available which have been designed specifically forhazard analysis of a wide range of lasers. Indeed, it is unlikely thtt such instruments will be made inthe future because of the great variation in exposure criteria for dif.'erent wavelengths and differentexposure times. Of course., such instruments could be made for each of the specific categories of lasers,but at present a set of these instruments would be quite expensive. L.i our experience a comprehensivi setof radiometric equipment for hazard analysis put together from present commercially available items wouldcost in excess of $20,000.

(2) Considering the present cost of equipment, one is forced to reconsider the necessity for hazardevaluation measurements and look for alternative techniques. Fortun.tely, we have found that mosthich-intensity light sources and modern lasers have fairly conriste-:t maximum output parameters. Becauseof this consistency of output and the uncertainties in present ex-.Osur? criteria, there is seldom a needfor periodic monitoring of a source. Quite often a source can be determined t- z.ave a radiant outputeither far exceeding or greatly below the present exposure stand•ards. Sensitive illuminance meters may beused to =easure permissible exposure levels for CW lasers operating in the visible spectrum. For example,10- W/cm2 at the helium-neon laser wavelength of 632.8 =m is 0.17 ft-cd, io-5 W/c 2 ;s 1.7 ft-cd, and

-• W/cm2 is 170 ft-cd. However, one must be sure that the illuminance meter is calibrated at thatwavelength and that the laser beam completely fills 'he detector aperture.

4. PHOTOGRAPHIC TECPWIJE S

a. lb this point we have considered only radiometric instruments; however, photographic radiometry-an play a valuable role in some instances, Determination of the effective source size is of critical!._portance in making a hazard evaluation of a high-intensity extended light somace. The radiance is ofpri~ncipal interest in s-uch an evaluation, and photographic tec.hniq-jes may be used to determine theradiance distribution of a source. It is important to determine the source size which the eye sees for-.-a reasons: T.o calculate the radiance and to calculat_ the retinal image size.

The effective source size can be accurately determined by photog:aphic methods. Generally, someagniithe source is r red. For small infrared laser diode arrays, a 35-M camera with a

-un lons an infrared film is quite useful at .'hort distances from tha source. At greater distancesa telephoto lers is often e--ployed. If the source is sufficiently intense, a high-resolution telescopew•ih 35-= came-a or Polariod back can yield excellent photographs. Unfortunately, rapia-processinginfrared fil, is no longer made. Is should be noted that ohotograohs should be taken at a number ofdistances as the optical ccllimator generally used in a laser diode system can chanae the appeirance of"the source at different viewing distances (see Figurp 6 for a series of photographi, taken at differentranges of a laser diode system).

c. one of the most important criteria for evaluating the ootential hazards from pul-;ed laser systems;s the outout radiant exposure. If the o.tput is above the levels considered safe for viewing diffusereflections, considerably more stringent controls must bee instituted. A rough guideline for determiningwhether the output is at or above these threshold levels can be arrived at through the use of appropriate.eat-sensitive papers or emulsions. If the beam reacts thermally with such paper, there is a possibilityof hazardus diffuse reflections. if the beam- does not thermally react with a specially chosen paper, itcan generally be assumed that the beaz does :ct produce hazardous diffuse reflections. Such papers canalso indicate emergent beam profiles for high-energy lasers. Heat-sensitive papers and their thresholdsare given in Table Ii.

d. Another possible photographic-radiometric technicqre employs photographic emulsions for radiometric-.easurements of radiant exposure. Absolute photographic measurements of bean profiles are quite complexand, in most cases, unfeaszble. -sarma curves (optical density of film versus the logarithm of the radiantexposure) given by film. manufacturers should be regarded is cnly representative of the type of emulsionsand sensitizing from which the characteristic curves were derived. If absolute photometric work isattempted, the tcllowing criteria must be met:

(1) The response of each emulsion hatch of a photographic material must be colibrated at theappropriate waveiengths under processing conditions which are identical to the actual measurementcondition.

(2) In addition, t),ere is the problem of making optical density measurements of the developedemulsion. Care must be taken with microdensitometers that read specular or remispecular density. Thesemicrodensitometers have a collection angle of less than 1800 and in general read somewhat higher thandiffuse microdensitometers. This is due to the scattering by the emulsio.i. The smaller the grain, the.ore closely the specular density approaches the diffuse density.

(3) The maxaium density of most coron emulsions varies between 2 and 3, and the dynrmic range cfexposure between the base density and maximum density is only one to three orders of magnitude. Becauseof this limited dynamic range, a special-purpose film with three er••lsions of differing sensitivities hasbeen manufactured. This special-purpose film is developed in separate stages as is color film.

e. Calibration.

(1) Calibration of all zadiometric systems is required periodically. The preferred calibrationmethod for the irradiance levels of interest (Table I) utili-es a standard lamp. Standard 500-H and

---------------------------------------------

4-7

1000-W quartz-iodide tungsten-filament lamps are available from se'•eral manufacturers with spectralirradiance and total irradiance calibration with an absolute accuracy of 2 to 10 percent. In ourlaboratory we use such a lamp to directly calibrate a standard disc calorimeter a,d a spectralirradiometer. We then place the monochromator between the lamp and the thermopile and use the disccalorimeter to measure the irradiance at a given wavelength of interest which must be known to calibratethe irradiometers which do not have a spectrally flat response. For laser w.velengths we can use a lasercalibration source. Same disc calorim'eters now have a built in-electrical heater for the copper disc anda known electrical power to this heater circuit results in the same temperature rise in tte disc as from aradiant power. This is termed "direct electrical calibration".

(2) The calibration of radiant exposure meters is more complicated unless the instrument behaveslinearly with changes in exposure duration. If it does, the irradiance standard and a calibrated shuttermay be adequate. There is a great deal of uncertainty in performing measurements of ultrashort pulsedsources, as from Q-switched or mode-locked lasers. Several methods have beer developed for measurament ofradiant energy output of pulsed lasers. A radiant exposure instrument designed to measure microjoulas/cm 2

or Less can be calibrated against such a radiant energy meter by measuring the output energy of a pulseilaser by two methods. The laser outpat energy, Q, may be measured directly with a ballistic thermopile orcalorimeter and indirectly with the radiant exposure meter by measuring the irradiance reflected from astandard diffuse surface such as .49CO3 or Mg02 and finding H from Lambert's law relating the reflected

radiant exposure (H), and the reflectance, o, to the distance, r, between the detector and the diffusesurface:

•r= (16)

for r >> laser beam e'.ameter, where e is the angle shown in Figure 7.

5. -XMASLmREm4MUT TECHIQUES

a The many techniques used in photometry and radiometry are far too numerous and complex to detailhcre: however, some com.on pitfalls deserve mention.

b. It never ceases to amnze me how often i encounter two different measured outputs of the samelasers obtained by two different l.."boratories. The answer is almost always a problem of "geometry" or theincorrect accounting for pump light or ambient light. Measurement of divergence at two points a fewm-eters away from the laser will often permit reduction of the error due to pump light and scattered laserradiation from the cavity as well as light in higher order modes. The use of narrow-band filters requiresenormous carc, and =eaqurement in a dark zoom or outdoors at night is far more accurate than usingfilters. Remember that measurements made in the infrared often require careful baffling and thac thesebaffles can heat up and emit significant infrared radiation.

6. Co:;.CVSTONS. Radiometric techniques and instrumentation are available to analyze hazards of exposureof the skin and eyes to high-intensity optical radiation sources. However, the cost for such equipmentremains xelatively high when compared to survey equipment 4vailable to evaluate many other environmentalhazards. Radiometric formulas 3nd manufacturer's specifications, when carefully applied, can often be anadequate substitute for measurements. If detailed information is necessary, however, at least some-eas-re.2ents are generally required. I have endeavored to si-r-arize the characteristics presentlyavailable in commercial readiometric equipment, but the audience must remember that radiometricmeasurement techniques can be quite involved and a knowledge of the effect of source reometry, filter, a.-dde'-.tor characteristics is required, as well a3 good instruments to properly perform accuratemeasurements.t

1 cm 10 cm 100 cm

Figure 6. Intrabeam Photographs of a Gallium-Aluminum-Arsenide Laser Diode Source withProjection Optics. Note that the entire aperture of the prcjecting optics is"flasher" at 100 cm.

4-8

TAzs.lf

Radiant Exposures Required To Produce a Visible Changeon Various Sensitive Media

Q-Switched Ruby Q-Switched NeodymiumLaser (694.3 nm) Laser (1.06 jt)

Threshold Saturation Threshold SaturationSensitive Surface (j/cm) (i/cm2) (/cma) i/cml)

Fully developed Polaroid print,black (coated or uncoatcd) 0.056 0095 0.07 0.2

Kodachrome II transparency.black (unexposed)&.b 0.17 0.21 -

Fully developed, fully exposed photographicfilm (Kodak Panatomic X)b 0.08 0.2 0.08 0 18

Dupont Lino-Write 7 direct writingphotorecording papere 0.01 0.05 0.02 009

Kodak Linagraph direct print papere 0.01 0.05 0.02 0.09Bkck paper used to pretect sheet films 0.22 ".22 - -

Black maskin- tape 0.07 0.02i 0.1 0.12Ca bon paper (Tru Rite type I)

Grade A. black medium tinishd 0.024 0.036 0.04 0.06Black printer's ink on white pape. 0.16 0.25

aBoth the color transparency and the black paper used to protect photographic film employdyes which have greatly reduced absorption characteristics in the near-infrared spectrum. Theexperimental arrangement used at USAEHA did not permit accurate measurement of radiantexpoures above 0.7 j/cm2; hence, threshold data at 1.06 A could not be obtained.

aloth the unexposed color transparency and the fully exposed black and white film had differ-ing thresholds depending on the side exposed. The thresholds listed are for the most sensitisefilm sides: the emulsion side for the Panatumic X and the nonemulsion side tor the KodachromeII.cThe visible response noted was a darkening of the paper. The responses of these papersvaried depending on previous exposure to ambient light.

'The visible response noted was a change in surface finish from a dull black to a glossy black.Note: The visible change is normally a lightening of the surface unless noted. Preliminary

measurements of the sensitivity of black Polaroid print film to non-Q-switched exposure in-dicated an increase in threshold of approximately one order of magnitude. Saturation levels areprivided only for the minimal tipe of surface change. In most cases more striking changesoccur at still higher radiant exposures.

RADIANT EXPOSURE METER

MI

BEAM SPLITTERBALLISTIC THERMOPILE

Figure 7. Arrangement for Calibrating a Radiant Exposure Meter Against A Calibrated Eallistic Thermopileusing a Single Mode Laser. Frr a lasez which changes modes and therefore changes beam polarization,the fraction of energy deflected by the beam splitter will change unless the beam splitter ispositioned at near-normal incidence and the ballistic thermopile positioned near the laser.

4-9

GENERAL REFEREVCS.

1. Bauer, U., "Measurement of Optical Radiations," The Focal Press, London, 1965.

2. Garbuny, M., "Optical Physics7 Academic Press, New York (1965).

3. Heard, H.G., "Laser Parameter Measurement Handbook, Wiley, New York (1968). 14. Walsh, J.W.T., "Photometry," Dover Publications, New York (1965).

-i

4 --

5-I

OCULAR EFFEClS OF LASER RADIATION: CORNEA AND ANTERIOR CHWER

Edwin S. Beatrice, M.D., LWC, Chief, and Bruce E. Stuck, Physicist, Non-Ionizing Radiation Division,Department of Bionedical Stress, Letterman ArnV Institute of Research, Presidiu of San Francisco,California, 94129.

The effects on cornea and skin of infrared laser radiation are based upon the absorption of energymrom these sources and the resultant "thernal" alteration of tissues. In the evaluation of these effects,

the two tissues must be considered together since both the cornea and skin are derived embryologicallyfrct surface ectoderm and mesoderm, and are most accessible to interception of radiation and absorptionof radiation. This presentation will center on the surface tissues of the huran organism, namely corneaand skin (9). The first phase will enphasize the normal anatorr and physiology of both tissues. Thesecond phase will involve the siperficial sumTary of those laser systems which mN interact with thesetissues, and finally the effects of alterations from these systems will be discussed.

During the past five years research into the comparative effects of infrared laser radiation andincoherent radiation on skin and cornea have been reported (1-9). The threshold levels necessary toprovide permissible exposure levels for porsonnel working with these laser sources have been primarilyderived from characteristics cf solar radiation in the infrared and extrapolation involving variables inthe "normal" environment from solar radiation. Characteristics of infrared laser exposure auid tissuethresholds are required for comparison with other incoherent source data for a more precise safe leveldetermination. The early data (1-4) was concerned with laser effects to ocular tissue. The comparativethresholds to skin must be presented and determlined (5-9).

The obvious transparency of the primate cornea is due, in iart, to the re-Vilar arrangement of thefibers of the substanria propria or stroma. Character-'stic of the histology Is -he epithelium,approxinately 50 microns thick, a condensed layer (Bowmn's merbrane) 5-1J microns, a dense stromal area(500 microns), Descemet's membrane (5-10 microns) and an essential monolayer, the endothellum (5 microns).The total corneal thickness is appro'dmately 550-600 microns. The cornea is vessel-free, with anextensive supply of pain endings. Alterations to the epithellum produce a rapid response toreepithelization and recovery. A surface tear film approximately 7-10 microns covers the epithelialsurface. A-thcugh viale, the entire corneal thickness is transparent. Laser exposure ma producetaporarly to permanent charges in this light conductivity.

Unlike the cornea which v-ries sllghtly from eye to eye or individual, skin thickness may .arywidely from 0.4 millimeters- to 5 millimeters depending upon location on the body surface. Additionally,pigmentation varies v dely across the body areas as well as among peoples. Basically, the surface layercorposed of nonviable keratinized cells (stratum corneumr) is 15 microns in thickness. 1he underlyingviable epidermis has a thickness varying from 50-75 microns and consists of str-aca including a thin,relatively homagene-cas layer (lucidum), diamond-shaped cells contaidrig keiatohyalin gra.ules(granulostn), polyhedral shaped cells (spinostm) and the deep coltw .ar epithelial cells (germinativum).Again unlike the cornea, cell division occurs in the deep germinal layer and progress toward the surface.Replacement of loss of cells .ist proress from deeper layers. Corneal morphology diverges ffro thesecharacteristics In that epithelial cells are viable and subsequent layere. appear to be autogenous in thedevelopmental cycle.

Currently, relevant laser wave..engths include neodymium (1.06 microns), erbium (1.54 microns),holmium (2.06 microns), hydroeen and deuterium fluoride (2.79 and 3.83 microns), and carbon dioxide(10.6 microns). These wavelengths correspond to abosolption characteristics which indicate surface ornear surface alterations.

When investigating the effects of monochronatic radiation, the Larbertian absorption coefficient,which is wavelength dependent, and surface reflectivitjy are important variables to consider. Theproportion of the incident radiation transmitted to a given depth of z is given by e-as where a is theLarbertian absorption coefficient. The hglier the absorption coefficient the larger the quantity ofenergy absorbed in a smaller volae. This can result in extensive injury to the most superficial layers.The smaller the absorption coefficient the larger the volume of tissue available to dissipate the energy;however, this pe-riits penetration to the underlving tissue structures. Heat conduction through thetissue must be considered as a means of alte-in,, the surrounding tissue. Since the ccrnea and skin areaqueous in nature, the absorption coefficient of water is a go-d first approxination. Th, absorptioncoefficients for the relevant infrared lasers are as follows:

Laser Wavelerkth Absorption Coefficientrmicrons) (cent imeter-! )

Erbium 1.54 19

Holmium 2.06 90

HF 2.79 4,900

DF 3.73 115

Co2 10.6 815

The direct utilization of these rrirbers enables the scientist to estimate the equivalent depth ofabsorption.

5-2

The biological Interpretation of data generated by the research into threshold levels Is aided byth.e uniderstanding of the interaction of laser energy and a degree of conprehension into the physics ofLa.±jertian absorption. T1he ultimate goal of the work Is to establish an extrapolation to protectionstandard levels derivedJ from. stbhurnan and vertebrate exposI.mrs. Additionally, the following areas will

oviean appreclat!on for those variab-les which are i.porta'it In this research. T1hey include:

1. Waveleng~th.2. !Zeam geometry both the spatial intensity distri~bu.tion ana the irradiance dialleter.3. Calibration and dosiretry.14. Pulse duration and repetition rate.5. Tissue character-st Ics.6. iindooint (tl-e after exoosure).7. cr::teria for alter-ation of tissue.

a. Grossb. !!lic,-scotoic

1)Lig'.t mlicroscopne(2) E-lectron -ticroscepe

8. St--atistical analysis of data.

Threshold da."age to the ezye from C0,b laser radiation is confined to the sure&per~f~cial areas of

cornea. Utitlizir~g the- absorption co--f--icient (81-5 centileter-l) . Nine-ty-nine percent of' the enera~te anzsor-bed within the first 55 micron~s. The ausorotion will occur then in In tear film (7 micr-ons)

an Pine Iial Il-ayer (50 microns). The criteria fcr corneal inju._y is the ore-sence of gray-white opacityat tne site of exa-c Th bevdwt n ltI. ircrsoe2 or fe xotv.'1e energyfoea& ex.aos.xre wasaried to deteermine the threshold level and data was analyzed Dy the technique of probit.ara-2sis. Data f;:- the floigpul~se duration andM lelsaegvnasflo:

Pulse D-zration E5(--Ill~seconds) (Watts/centimeter&)

i 8010

2 1485

10 72.5

1,000 7.7

5,0300 3.0

'..2cý,reaJ. allteration was corletel1.' reversible within 211-'8 hours. IPbre subtle thresholdcet~r~t~:~of a~te-atifn .cctir in the ilization of trypal: b1.ae stai4ning of exposed corsnaal tissue.

s qu-'e, wel-I know-Yn In cl. Callkrtols- techniques, allows the surveyCftecicavailysftiie corn-ea - endotheitr7I . At or ngear threshiold levjels, ext-ers1lve staining of the endottheJJirm (indicating

-. '*-~-ce'll. process), raises t;,e- s~,-tle cue-ztion of e.-4othelial repair and the additionalr.;;'mg endothe-lial cells at the center of the caell reared specimens que-stions the

s;I~a::itof tne exoo-sedtss.

2.'Zne tnresnold for -stc ed er u n radiation is aporoximately 20 jcules/cer~tinter . he- - crao'r~stlosof thi's radiatlon (tsoroticn o')efflcient - 19 centimeer) Indct hteeg

absroln (9ý, percent) wouldtopl I e exoece to occur In 2.42 niAllimerers. At above threshold levels, damage-.as cbser !-c, to- ocill thick-nesZ nen~etratir.g deep, into Descenwets -ýr-nbranee and endotheelial. layer. Some-nar-o teaterior ch:-. -!r were obse-nred ý'-e en-#o-"-nt was 24, houi3 post-.expost-re appearance of

The scye Itercmetaticrn of tnrsnh reels of' rad! a'tion to skin is based upon a di~vrgene rocls~cl~ad~ f ln~a ur a Al. or n-eaar the- threshold levels a system: utilized by the

in-vestigators involvI-es the olass-'fication of gradees of erythena (reddening of the exposure site) with or.itnsu-, concom~itant stre-tiri corneum= (white burn,) invoivre-nt or oblatirg of the surface layers. lur~theer

n~so~hcogCal anp! -'s of exposta'es Is ;-ha'pered by the presence of chronic der-.mtoses of por-cine skinas we-,- as preparation artifacts p-rior to exposu~re. Tedata pr-esented for porcine skin exposures observed

* ~ 'osl::at Ihourand 2L noxz- os-exposr is as follows:

(mdilliseco~nds) (watets/centk m4,eter

4-.3 247

39.0 37.5

220 13.6

300 10.6

30 7.6

710 4.7

336 3.7

'1'100 1.7

5-3

SJti-.zing sirsiar exposure criteria, porine skin was e.xpsed to erbium. radiation Ln the range betwecn8 Joules/centimeter-- and 80 Joules/rentimeterý. A total of 148 exposures were made in five pigs. Noerytheema was observed in any of the exposures at the 24 hour level for anry of the enerc densities rulingout the analysis of the ED- level.

In .d..siretrj, it w-as necessarnr to focus the output ener•' onto the tissue to permit energtdensitic of producing corneal alterations.

T in•-.retation of corneal expcsu-e experirments mandates further evaluation of the significance ofthe corneal endothelial altervtions. In addition, the effects of low level continuous and repeated pulsedSraalation for these infrared laser sources i' necessary. The co-pletion of these studies will add vitallnfo.ration to the knowledge of corneal response mechanisms to laser radiation. The effects of exposureto continuous exposure within present safe level envirorrients n-IMst be kept in mind for the existentporsiblity of long term. chronic effects due perhaps to cum•lative absorption, or the future sequelae of-direct effects on corneal endothelium, or stratun genminativu-n of the skin. It must also be understoodtha.t the present expertise and research will continue to modify whatever safe levels are postulated.

i. Fine, B., Fine, S., Peacock, S. F., "Preli.inar- Observations on Ocular Effects of High-PowerSContinuous 002 Laser irradiatic.," Am J OohtlhallmoL Vol. 64, 1Jb. 2 (August 1967), pp. 209-222.

2. Lelbowitz, .M., Peacock., G. R., U. S. Ar..-i; !Medical Research Laboratory, "Corneal Injur7.roduced by C02 La-ser .adiaticn" (August 1968), Report 787.

3. Ca 1hell, C. J., littler, M. C., Bredereter, K., et al., "Ocular Effects Produced by Expe-rimentalLasers," Am J 2,-hthaluol, Voi. 66 (october (1968), .-. 604-61a.

%. .assiliad4s, A., Zweng, H.. C., Peppers, N. D., et at., "Thresholds of laser Fye Hazards,"A.ch En'viron Health Vol. 20 (Feb."'-'y 1970), pp. 161-170.

5. Ha-rdy, J. D., .,-.s-he.ie-m., C., ".Radiation of Heat from the Hman Body Versus the Transmission ofInerared Racdiation T.hrough Skin" j Clin Invest, Vol. 15 (1936), pp. 1.

6. ne.nriqes, F. C., Jr., "Studies of .-he..l Injur7 Versus the Predictability end the Siguiflcance

of T.-he._...-ally induced :-Pae -rocesses Leadinrx_ to irreversible p.idermal .nju.-y," AMA Arch Pathol, Vol. 43•'• ~ ~ ~ ~(1-7), ppo>.--z!!9 . .,-•.,>..,

7.Rrawnell, A. S., Parr. 2. H., S/kn arnd Car.on Dioxide Laser Radition," A-ch Environ Health,Von. 18 (19609), pp. 1137.

.• K .s, J. G. , 5Helevig, E. B., Hayes, J. R., "Effects of Lase•. Radiation on the Skin," NezemRecord (2b.9-6er 2S65), p.- 152.

9. Brc..mneU-!, A. S., Stuc., B. E., "ODcular and -Skin Hazards from 002 Laser Radiation," Proceedingso' t Ar--m- Cn!o.-erenrme (June 197h).

Ia

AAIN

-A-

-- I

.8l/ 1. QP.7I 1v- ., . .

WAV Ir~a II j

Sp-trlTaimotn TIo l fo -aEy

5-5

r . .. . .-- ..- . 'vV.½_ ...

_ A

I2

Serial sections of rhesw; cornea (A) and porcine skirn (B). .Note corneal epithelium and interkovenstroIa pattern, as -'ell as dermis and epidermis of skin.

Carbon dioxide exposure to rhesus cornea -at 65 •-tts/centimeter2 - 100 milliseconds. One hour .•ost-

exposure. Stromal clouding is evident in this zime frame.

6- 1

--:.e." in. RŽtzio Faiat Io v.' s 1or i, Ži of Bla-PI-ld-Ic 3t.-ss,!.Žtterman A:rZ.Lntt.; of Fsearch, ?rns:di ofaicso %~o~~,9a9

A srr~-:'-f t~eenzoc~tz -'n esta:ot~!.Ir thresl-old levels for vlistbl- a.-Yd ne-ar !nfrareO. laser~~~~~~~~~~~1. to- Irmt :ý!!. sz.. ~e fn~_ e~~ihsocy \nuscopy (ophthalmcscoplic),

o:'ocor.o t: ~t. :t.±ri. Z'ne:l~iarycorrlato:.11' bee mad'- ;*tnbehavioral-n ' 10rs al' v -EIrý 1.OXST -. w 1:i :I lc a -a a n-eetd focr c o ntr.d s tu tit;n t: e Ln te rfa ce L--txe e ngrcss allraor (opacl*:) ndt :,,-- a ea.:atlon.

.. .esal!S afe qo r-tiniz 1ev'els fo-r the use of 'asers,- In tne rilitary and clr'I~a:n-s-l-z r~.ý- zes-*.:eo -'n eternsive t-'a err.-et nvolvinz sopnzr. lratou Subjects. Dx-'r

* the pae:t --. en r extens!-:- r-tsex--- -:. ne ffe-cts o.* I~~eTsei- Irmaulation on t'e- pri.-ate andv.oaereti'na- nas !:Eenor-ut.ii rz oca*trieýs (I-10). A I amal rexiew of tine "-Ornz"f~-oso.and . oancr:;tinetlriate weia~ill rrov~ide a zoase n-=. whichleato~ can ber

:ec:r-e4 c.ohr ~~~a: ani enb'rz p~irroesses affecting the,- in--

:;fe~o L:rat7e ev~e ischarac-er~zed ray the- optic dio ryon-

ofc.z viit-an'hzrd fomn 7..ase: raa 7o. he macilla in prime-tes is relatively larige.naasz'es ' -'n ,i~a~a:er -a-n týne folvera a~r-,xL-at-eiy 'C -253 micr-ons. :n theP fov'ea, the-

2:o:' !zo'o ~ It. ir'aatest ',a:oocxlmatei::1t7ZC per iIntr)

;2J zz~. ~ nxt ntf :n-Žrtln. (sensor: reofr-.a' are"r -ret .3L:2 that the vie- of theo st:o-... rext in ar :,eato C~f t-'-- lalrsnoiZer.Att

asozs :o-Se:-atone. ,a~ --. te e .±Ile Z ou'qI:Stlons as to retlinal- sens;5!it.v!ty may--------------------------------j--'O5ý *-ssue. For tinis '~'~ratt.*ýonrC ;s

- ............ ... .~ o incom"ing 114.t, tý- kerola: ~.e. -.hi !- tc

-e an n -az-r ..rcez~ -ý =nn~iles of therI-l rig-ernt

-.. a -z -. f-................ n- At--

fna- ne s rr a %-r- a 'e a:--A

. . . . . .. . . . . . . . -- - - - - -- t- -. ..ýe. .ns

Sr- -n'- '~ **~' ~C*' C

Tedata in Tasble 11 shows the relationship for an argon source (LJ88 nanomters) for two retinalirra'4 4 ',,.ce diannters (30 and 200 microns).

SeveraL techniques enable Investigators to develop and access threshold data by utilizing the lightidcroTope. Corarilsons of equivalent exposures by direct observation and lig~t microscopy for argon,

514.51 rnanonieters, 125 milliseconds and r-my, 694.3 nanomrters, 30 x 10-9 seconds indicate a reduction in%Jlelof3pectusnthe serial sections and subLtle FPE endpoint. "Me limit of detectability

at theem~pooia level- is, of course, the electron microscope wheree alterations observed for Q-6witchedr-,by by E- techinIques Is ten times lower th~an the clnically observable MED_. level.

DzF.v !-thýer olc-pof lora level Q-switched :-uby c-x pstues ten times below, ED50 levels Indicates notonl na~ce ul-us-rururl ate-atcrs bu pes.4tene o thsealterations for un to 18 monthsafe

In.:1:a e=ýos.ure. uordoahcretinal technicues of simila-r expozurees Indicate the blockade ofprmcteln S.-n:;n-s~s in retina -nds (radloleucine).

Thres:nold orecitv levels also have been shown. to depend u-non exposure site and retinal pigmntation.:he macala has been repooted *ýo bee nre sensitive, by a factor of two, than par-ancular sites. increasedsens~tIvity, Is found In tne area Inrdaeytamorall to the racula at both argon and ruby woavelengths.As5 exp'osurtes are -laced inrezn~-& 'of-ax1 s, t;eopticall limitation~s o'teeye degradetheLng

~x~lIy, rouc~~gaberrn-t on.acities (if tihe Lear. is ciiaracter~stically ', central exposuxres produceC~rcu2.ar opac:ites; off-aPxis ex-posures produce ellipsoida1 opacities).

These olocal' experirents area rcutinel:. nzerforned in teanesthe&tized, ecretropic dilated ani~ral eye.::seccrnditorns' ,wh'le cnrcontr-ollied exposureý techniqu-es, a-re far recomved fr-om t;he skrilaticn of

Te enera~l anestnetic used depressc -.he central nervous system, reducing the ability to detect the-L-ora:s neurzall sIs uhcn~:-P indicate al-ers--ic., in th-e ohoitccherd stry at levels far beelowe opacity

.. leraev.els whýIle theý first of opacity ray be irortaint. The long terz, significanrce ofabsence of' cracity in tne coss~it e presence sf deprezsed electrophysiolongical sli;,als ilbee of rore

ZI;=iance.

AZt týne rct-Inal, :tye~ re a"Arn, ;.n in the.' p~c and neural eensaoite1..- L r em-Zrse a_,:era:ions to. any or all of, the n reillyrs mia4J pauce

pe:a~nnt iz: Fr_2e -vs gat s f-du fmtr that intense-aser exr~osurt' can se*.erely-ý- alter tnt' sa proez-?s . F:-veal ex~sr abovce tl reshold can *orocuce

_ý,:maen- r-ag5I _,ýhnaxi:-al v.,s.:al ac an- ~c,ýor visl, ). ?er~nt zane in the vIsaLk~Jasn': ad:- ras te to,'. !t;-c : -hfcveal irrasi4ation. itte~oc t seer-S tnat

tswsrýe case- for *r-:og cr_`teria, as ftnc:'-na2 critýeria are- refin-d, l-aser enerýj levels at

?'z .. rc -isr~rt vii, Zo '.l cl.;'nvest~gat~ors :f charnges :--. -.Orphoiof' cal andfc::*.alc :-Žr*a ar- ter en:;er- :ev-.FV ..... ..cn ffects '_Ccc.%111 be cbtained: at levls uc

Fr; zn 3 e *ýof :§:Ž±i-_n- and .7:-:; R-t~a -. 7 Thresnolds, 7LInvest

e- C., "mctS ýf *-r-- 1aser Fadl~a.Ion: Hiintopatholoe, of'- .~ '~- A~C ~th~l *Xt..~Ns.. !-=e 1a,, p_. 1267-1276.

7 -Vn1 4, ('95) Pp. 390.

5. ar ., *~~~~. - :.aser ~ Acta _phth&L-r01,Vo.43(95,p.9.

.................... A- ., en-~ t~.., thequenv-Doub led 4-dtir-iuZ_ ,z ntý La-ýser"k .A ~ 3 (Yrh1971), np. 63-1-638

7 ~ -. , ~ , *t:~.:.a~-r:rmadla-lcn ta.-Lter =s't rton, A pplied pŽt cs,

6. F.are .2,riscn, 'D. , --rarazao L -a.c hrsholds as a F'.=ction of Lzage~~~~~2 Arh-.t. .:v--- r 22--426.

Ad.- ,:~ Ce .'Betra:~1r~-t.UA:rtc-sFeue by. Extrerely !a.; Levels

_'tal. ,':zta an"; Tha1_fb Sto soclaled wih:aser Fiadiatican," 61od Probi

-~ ro~:iuas hvebeen -4-I'lu i~nt -.-'-.aration of the e, ur notes for the lecture series.am-: L-; ac ,....,.., , C...at 2;ViskW Lstitute a R se c

~d.'- fo hs tc--_ s andl assistancce In t%& :oarton of the lecture notes , and Dr. 'Harry Zwick,e~eacn hol~ist Leteman ~...~te~f -. ~acnwhoprovidf-Js valuable assistance In the

'frvestr~V ofth >c~r noenan,.d the addedý- wci n . tnes:covso of' v-1sion.

6-3

F'undus photograph of rhesus retina one hour after varying exposu.-s to Q-sitched rby radiation.Total energy at cornea is indicated for each exposure.

1A

•rrmal appearance of rhesus retina as observed ny light microscopy.

64

A

LW

B

C

-Vý.

(A) Light rniCrOgraphs of acute Q,-sh'itched neod-pli n1 retinal expoostires (1,320 microjoulcs). 4ot edisrup~tion of pigment epithelia] cells and vacuole in srtIrilsae

(H) Note distortion and disarray of innacr and outer segwnnt: of photoreceptors and changes inpigmcnt epithelium (478 microjoules).

(C) Eilectron ;1hotonicrograph shows altered photoreceptor vesicle and whorl formation not evidencedby direct observat ion or lipht microscopy (40 microjoulc..).

7.1

DETERMINATION OF SAFE EXPOSURE LEVELS: ENERGY CORRELATES OF OCULAR DAMAGE

R.G. BorlandRoyal Air Force Institute of Aviation Medicine.

Farnborough, Hampshire, United Kingdom.

SUMMARY

Practical, but safe criteria, for use of laser systems require an understanding of tissue damage,and three techniques have been used to define energy correlates for safe exposure levels. Inspection ofthe eye by optical means (ophthalmoscopy) has pr~vided the basis of min. -al visible damage for many studies,fluorescein angiography has explored the disturbance of the blood-retinal barrier induced by laser irradix-"ation of the retina, and microscopy (light and electron) has attempted to define the energy correlates forminimal structural change. The detection of damage is a form of quantal response and the determination Afthe threshold level is normally based on the energy o: power which will result in a given probability ofdamage being detected.

The energy correlates of damage depend on wavelength, pulse width or exposure time, repetition rate.tissue type and pigmentation, and ocular quality. This complex relationship necessarily limits experi-mental research to laser systems of special interest and so the interpolation of data to formulate o.erall.afe exposure levels is necessary.

INTRODUCT|ON

Until recently laser operations tended to be confined to the laboratory and protective eye wear withengineering controls provided adequate personal protection, but with the increasing use of lasers for bothmilitary and civilimn purposes in less well controlled circumstances careful consideration of safe viewingdistances is essential. The initial step in the establishment of safe exposure levels is to determine the

-; energy correlate of threshold damage. Only two structures need be considered at risk from laser radiation -

the eye and the skin - but the eye is of primary concern, as even a minimal damage to the eye may impairvisual function.

DaIage Mechanisms. The primary event in any type of tissue damage caused by laser radiation is the absorp-"tion of energy in the biological systes. The absorption of the photon of incident energy occurs at anatomic or molecular level and is wavelength specific. The process can result in the general increase int he vibrational energy of the molecule which gives rise to an increase in temperature (thermal damage) orcan change the inter-molecular bonding within the material and give rise to a chemical change (photo-chemical damage). Both processes can change the tissue components (denaturation) so that the biologicalfunction of the tissue is impaired. Far infra-red radiation usually produces thermal damage, while thefar ultra-violet radiation leads to photochemical changes. Between these regions the near infra-red andvisible light may produce either type of tissue damage (Fig. 1). Within the radiation band 400 - 1400 nm

A the eye transmits and focuses the energy on to the retina and so concentrates the energy incident on thecornea by several orders of magnitude. Outside this band the eye is virtually opaque and the energy isabsorbed by the cornea, lens and transparent media. It is advantageous to consider threshold studies intwo parts ie. damage to cornea and lens damage to the retinal structure.

CORNEAL STUDIES

UV Radiation. At the present time there is little reliable data on the biolcgical effects of UV laserradiation. Vassiliadis et al (Ref. 1) using a Q-switched frequency dcubled ruby laser (A= 347 nm) demon-strated that the primary absorption site at this wavelength was the lens. Maclean et al (Ref. 2) used ahelium cadmium laser (,\= 325 nm) but did not observe any lenticular cnanges. While there is a lack ofexperimental evidence of damage from laser sources, a considerable amount of data has been obtained usingconventional UV sources. As early as 1916 Verh-,eff & BeliCRef. 3) published a comprehensive review of JVhazards and showed that at wavelengths shorter than 305 ma damage was to the cornea and corneal epithelium,with confined widespread changes in the lens capsule. Cogan & Kinsey (Ref. 4) demonstrated the peaksensitivity of corneal tissue at an incident wavelength of 288 na and determined the threshold dose to be5 miliijoiles/cm2 . They also stated that the tissue damage was dependent on total energy and not on therate of absorption. More recently Pitts (Ref. 5) in an extensive review of the problem of UV radiationand a detailed research programme has quantified the effects of UV radiation within the band 210 - 330 nmin the threshold production of photokeratitis.

Middle and Far Infra Red (1400 rm - 10(l0ja. within this waveband there are three main lasers that are likelyto be of importance. The CO2 laser at 10.6 microns, the holmium at 2.06 microns and the erbi= at 1.54microns. Radiations from all these laser types are absorbed by the aqueous content of the cornea and thepre-corneal tear film.

Fine et al (Ref. 6) have studied the long term effects of exposure to 10.6 micron radiation and reportchanges to the corneal structure and appearance at levels of about 0.5 watts m-2 for periods of 5 minuteswith no damage after exposure to less than 0.1 watts cm- 2 for periods of up to 30 minutes. Campbell &Rittler (Ref. 7) report the production of just detectable damage from the C02 laser after exposures of3.85 watj/c/m2 for 3 seconds and Leibowitz & Peacock (Ref. 8) studying shorter exposure times give a

threshold of 15.6 watts/cm2 for an 6O millisecond exposure. Peppers et al (Ref. 9) report figures ofthreshold damage of 18 watts o- 2 in 70 milliseconds and Gulberg & Hartman (Ref. 10) studying theproduction of a blink reflex quotes a formula - Q (joules cm- 2 ) = 1.26 t (secs) for exposures within thetime range 0.01 to 5 seconds. This is in reasonable agreement with the work of Borland et al (Ref. 11)who state that 'me corneal threshold level is 6.9 watts cm- 2 for a 70 millisecond exposure. This not

K L7-2

inconsiderable spread of experimentally determined threshold levels can to a large extent be attributeC tcdifferences in the experimental techniques ie dosemet-y and the matter ef detecting minimal damage.

No threshold data is available for the holmium laser and only one group, Bresnick et al (Ref. 12) htvestudied the erbium laser for which they report a corneal threshold c 17 joules/cm- 2 for the 50 nanosecondQ-switched pulse.

RETINAL STUDIES

Visible and Near Infra Red Threshold Studies (400 - 1400 nr). Thc majority of lasers of military and civilimportance radiate in this part cf the electromagnetic spectrum (except CO2 laser at 10.6 microns) and itis hardly surprising that the majority of laser safety scudies have been concentrated within this waveband,where the retina of the eye is at risk.

In considering threshold damage to retinal ttssue four main factors must be taken into accot.it. Theseare - a .efinition of threshold, wavelength, exposure t:-e and image size.

Damage to retinal tissue following laser radiation can be detected in one of four ways of whichophthalmoscopy is by fir the most common and has provided the basis for %ainimal visible damaoe' studies.The development of an ophthalmoscopically visible lesion is time depensdent and although a one hour postexposure criteria is commonly applied in experimental studies some lesions may not become visible frr upto 24 hours post exposure. Opht-Almoscopy is at hest a means of establishing a baseline for comparingexperimental studies, the detectability of lesions being dependent on the type of ophthalmoscope used, theskill of the observer and the image size being observed. A secDnd and more sensitive technique in thedetection of minimal damage is that of fluorescein at.giography which can demonstrate small disturbances ofthe blood retinal barrier induced by the 'thermal' reaction to lazer irradiation. -luorescein angiographyis three or four times more sensit've in detecting lesions than ophthalmoscopy but stiil represents arelatively severe level of damage in the pigment epithelium and receptors and while there is evidence thatseveral months post exposure the site of a fluorescent lesion has showed histo:ogic evidence of structuralrecovery, there is no direct evidence that the functional integrity of the retinal organisation has beenrepaired. It is .nerefore necessary to use electrormicroscopy as the final endpoint on which to base safeexposure levels in order that both permanent and temporary damage to the retinal structure may be avoided.

Some stud..es, McNeer et al (Ref. 13) and Davis & Maultner fRef. 14) have reported on the effer s oflaser radiation on the electrical responses of the eye. Howevei, although the energies required to produceminimal electrical disturbances are less than the ophthalmoscopic threshold levels they appear to be higherthan the electronmicroscopic threshold levels as determined by Adams (Ref. 15) and Landers et a: (Ref. 16)ano Borland .'in preparation).

Wavelength. Fig. 2 illustrates the percentage of the radiation incident on the cornea that is absorbed inthe pigment epithelium and choroid of the rhesus monkey, the Lost commonly used experimental animals,rabbit and human. Radiation within the band 400 - 700 nm are also absorbed by the photo-pigments withinthe receptors and evoke a visual response and although in general it is thp melanin granules of thepigment epithelium wich provide the ab.•orption site for the incident radiation, prolonged exposure toradiation within the risible part of the spectrum can induce ret.'nal damage that canno- be relited to thenormal thermal damage mechanisms.

Time. Ocular exposure to laser radiations may extend from continuous viewing over a period of severalminutes or hours to a single mode locked pulse lasting fractions of a picosecond. Not surprisingly overthis range of exposure times the mechanism of damage does not remain constant. Sperling tRef. 17) hasshown how prolonged exposure to the argon laser radiation can produce permanent impairment of the spectral

. response to the rhesus monkey eye while rNoell et al (Ref. 18) using rats and Marshall et al (Ref. 19) usingpigeons have demonstrated how even fluorescent lighting at moderately high levels can produce profoundretinal degeneration over periods of several hours. This mechanism of damage is not yet fully understoodbut it would appear to be a 'poisoning' of the receptors by a prolonged generation of Zhe by products ofthe photo-:hemical processes. For exposure times of less than 100 se- down to times as short as 10 microsecinds the damage to retinal structure is due solely to localised tissue heating. Several research groupshave studied this time domain and the Stanford Research Institute (Vassiliadis et al), the Joint LaserSafety Team (Beatrice et al) and the Medical College of Virginia (Ham et al) have used a wide range of laserwevelengths and exposure times and have shown a remarkable degree of agreement.

Studies at times shorter than 10 microseconds have been limited to the Q-switched ruby and neodymiumsystems and to the pulse argon system. Data from the main research groups again show a remarkable agree-ment but the data when compared with the results from the longer exposure periods indicates tl-at adifferent type of damage mechanism may be present and it has been suggested that the damage observed iscaused by acoustic transients generated by the rapid expansion of the retinal tissue as a result of thesudden temnerature rise induced by the very short pulses, (Marshall et al, in preparation).

Retinal Image Diameter. The retinal image size and the corresponding energy distribution are importantfactors in determining the time/temperature relationship involved in thermal damage studies. For an idealeye accommodated to a laser wavelength, the image size will be equal to the product of the eye's focallength and the divergence of radiation incident on the cornea. As the beam divergence reduces, that is thelaser becomes an effeutive point source, the image size reduces until theoretically limited by diffraction.

The distribution of energy in the plane of the geometrical focus is the well known Arey disc surrounded byconcentric bright and dark rings. The diameter of the retinal image across the first dark ring is givenby the formula 2 -4 42k f where)is the wavelength in ml :rons, f is the second focal length of the eye in mm(22.9 for human cse), n is the refractive index of the ocular media (1.334) and d is the pupil diameterin mm, thus for a 7 mm pupil the perfect 'eye' would be capable of producing a retinal image diameter some

6 wavelengths in diameter. However the eye is not a perfect optical system and contains aberrations. Tha

eye is unlikely to be corrected for the wavelength of the incoming radiation. The effect of sphericalaDerration tends to compensate for the reduction in image size due to diffraction with increasing pupil

7-3

diameter, an empirical approach to the point image spread function h&s been made by Mayer et al in Ref. 20.A more cxacL approach to the problem of the distribution of energy across the retinal image and the limit-ing retinal image size is to cozsider the Optical Modulatior. Transfer Functio- which quantifies theperformance of any optical system in terms of the ratio of the corneal energy distribution ti the retinalenergy distribution. Several researchers, Campbell a Gubisch (Ref. 21), Westheimer (Ref. 22) and"A. van Meeteron (Ref. 23), have studied this problem in the living human eye for both white and quasi mono-chromatic sources, bu° ac yet however no stu'les have been made on the modulatio.i transfer function of thee~e of the rhesus monkey or rabbit, the two species commonly used in the determination of retinal thresholdlevels. In the rhesus monkey Stein & Elgin (Ref. 24) have reported in vivo estimates of minimal image sizefor white light of some 30 microns while Vassiliadis et a. (Ref. 251 zepirt est.mates of riiimal imagediameters oZ 90 microns at the neodymium wavelength and some 50 micron.3 at the ruby wavelength. In therabbit eye Ham et al (Ref. 26) have noted how the poor optical quality causes an increase in the apparentenergy per unit area necessary to cause threshold lesions with decieasina image size. Additionally, Jou.es& Fairchild (Ref. 27) have shown that the Optical Modulation °iransfer Function not nnly limits the imagesize and degrades the retinal image distribution but also affects the ab'lity to detect dama3e viewed..hrough tee reverse optical path.

Whea. the observed damage is thermal in origin the basic considerations of heat transfer indicatethat heat is conducted away more slowly frc the central portion of a large area irradiance that, for as• 'let area and conseq" !ntly it is to be expected that a lower threshold for damage will be found (in termsof retlnrl radiant exposure) for the larger irradiance diameters. Several mathematical models have beenput forward which prowide -xcellent agreement with the experimental data for image sizes of trom 10 - 1000microns and aver a time scale of from se eral microseconds to seconds, and a complete review of thermplmodelling has been presented by Wolbarscht (Ref. 28,. Beatrice & Frisch (Pef. 29) have demonstrated howa similar dependence of threshold image diameter is found for both 1 sec argon exposures and 30 nanosecondQ-switched ruby exposures. As yet no satisfactory model has been put forward wh'ch will explain all theobserved resulzs especially those related to the Q-switched time domain where the retinal threshold level"is a.so dependent on image size which is contrary to the basic principles of heat transfer. This supportsthe view that damage to retinal tissue by Q-switched irradiation is not purely thermal in origin.

Determination of Threshoid. The experimental determination of a threshold level involves tde detection ofthe abseu.:e or presence of a respinse in a subject to a known exposure level or stimulus. The character-istic response is said to be quantal if the occurrence or non oc.currence depends on the intensity of thestimuluis. For a particular retinal si .e there will be a level of s'imuius below which the response doesnot occur and above which the response always occurs; this stimulus level is called the threshold. Thethreshold level will vary between retinal sites and between subjects within the population studied. Theanalysis of quantal response data is best carried out by a statistical technique called Probit Analysis,Finney (Ref. 30). :esults are presented as a linear regression line of damage probability plotted againstthp logaritha. of the dose. The dose correaponding to a 50% probability is termed the ED5 0, and is commonlyquo.ed as the "threshold level". Probit analysis also provides the normal statistical evidence as to thesignificance that .an br placed cn the resuits. Fig. 3 shows a typical regression line, with 95% confidencelimits on both EDSo value and slope.

The data is for Q-switched neodymiim irradiance of the rhesus monkey eye, with an image diameterestimated at 30 microns.

Probit analysis also allows a comparison to be made either between the effectiveness of differentstimu.i (laser parameters), or between differences in the response within the same population to the samestimuli. Fig. 4 shows the regression lines for damage as determined by ophthalmoscopy and fluoresceinangiography in response to Q-switched neodymium laser radiation. The ED50 values are significantlydifferent, and show that fluorescein angiography is a more sensitive technique in detecting threshoLddamage than ophthalmoscopy.

A single experiment to determine an ED5 0 level for a par.icular type of laser radiation does not in Iitself provide a basis for the derivation of safe exposure levels. However, an understanding of the majorfactors affecting damage together with data from experiments invclving similar laser parameters areessential ingredienzs in the formulation of safe exposure levels.

REFERENCES

1. Vassiliadis, A., Zweng, H.C. & Dedrick, K.G. Ocular Laser Threshold Investigations. StanfordResearch Institute, Report 8209. Menlo Park, California, (January 1971).

2. Maclean, D., Fine, A. Aaron a Fine, B.S. Preventable hazard at UV wavelengths. Laser Focue 7: 29(April 1971).

3. Verhaeff, t.H., Bell, L., & Walker, C.B. The pathological effects of radiant energy on the eye.Proc Am Acad Art a Sci 51: 630 (1916).

4. Cogan, D.G. & Kinsey, V.E. Action spectrum of keratitis produced by ultraviolet radiation.Arch Opl-thalmol 35: 670-677 (1946).

5. Pitts. D.G. & Gibbons, W.D. The human, primate and rabbit ultraviolet action spectra. Universityof Houston, College of Optometry, Houston, Texas (March 31, 1972).

6. Fine, B.S., Fine, S., Feigen, L. & Machen, D. Corneal injury threshold to CO2 laser. Am J Ophthalmol

66: 1-15 (July 1968).

44

17-

7. Campbell, C.J. Pittler, M.C. The effects of lasers on the eye. Ann NY Acad Sci l68(3):627-633(February, 1970).

8. Leibowitz, H.M. & Peacock, G.R. Corneal injury produced by carbon dioxide laser radiation.Arch. Ophthal. Vol. 81 (May 1969).

9. Peppers, N.A., Vassiliadis, A., Dedrick, K.G., Chang, H., Peabody, P.R., Rose, H. & Zweng, H.C.Corneal damage thresholds for CO2 laser radiation. Applied Optics 8:377-381 (February 1969).

10. Gullberg, K & Hartman, B. C02 laser hazards to eye. Nature 215:857-858 (August 1967).

11. Borland, R.G., Brennan, D.H. & Nicholson, A.N. Threshold levels for damage of the cornea followingirradiation by a continuous wave carbon dioxide (10.6 ur) laser. Nature 234:141-152 (November, 1971).

12. Bresnick, G.H., Lund, D.J., Landers, M.B., Powe, J.O., Chester, J. & Carver, C. Ocular hazards of aQ-switched erbium lasers. DHEW. PHS. BRH PubliCition. No. BRH/DEP 70-26, "Electronic ProductRadiation and the Health Physicist" - Health Physics Society Fourth Annual Midyear Topical Symposium,January 1970. DHEW. PH5. Bureau of Radiological Health, Rockville, Maryland 20852 (October 1970).

13. McNeer, K., Ghosh, M., Geeracts, W.J. & Guerry, D. III. Electroretinography after light coagulation.Acta Ophthalmol (Sunpl.)(Kbh) 76:94-100 (1963).

14. Davis, T.P. & Maultner, W.J. Helium-neon laser effects on the eye. Annual report no. C106-59223,Contract DADA 17-6v-C-9013. EG&G, Inc., Santa Monica Division, Los Angeles, California 90064(April, 1969).

15. Adams, D.O., Beatrice, E.S. & Bedeil, R.B. Retina: ultrastructural alterations produced byextremely low levels of coherent radiation. Science 177:58-60 (January, 1972).

16. Landers, M.B., Beatrice, E.S., Byer, H.H., Powell, J.0., Chester, J.E. & Frisch, G.D. Determinationof •isible threshold of damage in retina of rhesus monkey by Q-switched ruby laser. MemorandumReport M 69-26-1. Frankford Arsenal, Philadelphia.

17. Sperling, B.G., Harwerth, R.S., Mabry, J.H. & Landis, D.J. The effects of laser radiation on receptorfupctfon in human and primate eyes. Report U. T-GSbS-DADA-2. The University of Texas Graduate Schoolof Bicmedical Scieoces, Texas 77025 (April 1970).

18. Noell, W.K., Walker, V.S., Aang, B.S. & Berman, S. Retinal damage by light in rats. Invest Opnthalmol5:450-473 (October 1966).

19. Marshall, J., Mellerio, D. & Palmer, D.A. Damage to pigeon retinae by moderate illumination fromf.Lorescent lamps. Exp Eye Res 14:164-169 (1972).

20. Mayer, H.L., Frank, R.M. & Richey, F. Point image spread function for nuclear eyeburn calcalazions.Report No 56321. Defense Atomic Support Agency (June 1964).

21. Campoell, F.W. & Gubiscn, R.W. Optical Quality of the Human Eye. J Physiol 186:558-578. (1966).

21. Westheimer, G. Optical properties of vertebrate eyes. Handbook of Sensory Physiology. VII/2

449-482. Edited by M.G.F. Fvortes.

23. Van Meeteron, A. Calculations on the nptical modLIation transfer function of the human eye forwhite light. Report No. IZF 1973-2. Institute for Perception, Netherlands. (1973).

24. Scein, M.N. & Elgin, S.S. Measurement of retinal image diameter for laser radiation in the rhesusmonkey Final Report Contract No. F41609-68-C-OO33-6301. USAFSAM Brooks AFB, Texas (1970).

25. Vassiliadis, A., Rosan, R.C. & Zweng, H.S. Research on ocular laser thresholds. Stanford ResearchInstitute. Final Report Pro~ect 7191 (August 1969).

26. Ham, W.T., Williams, R.C., Mueller, H.A., Guerry, D., Clarke, A.M. & Geeraets, W.J. Effects of laserradiation on the mammalian eye. Trans. N.Y. Acad Sci. (2), 28:517 (1965).

27. Jones, A.E., Fairchild, D.D. & Spyropoulas, P. Laser radiation effects on the morphology and functionof ocular tissue. Second Annual Feport Contract No. DADA-17-67-C-0019 US Army Medical Research andDevelopment Command (1968).

28. Wolbarsht, M.L. Editor. Laser applications in medicine and biology. Plenum Press, New York (1971).

29. Beatrice, E.S. & Frisch, G.D. Retinal laser damage thresholds as a function of image diameter.Arch Environ Health 27:322-326. (November 1973).

30. Finney, D.J. Probit analysis (3rd Ed.), Cambridge University Press. (197A).

• ~ ~ ~ ~~~~~~~~~~~~~~- ---- ------. .. . _. -;..... .... .... _T: • ....

7-5

I

Short ultraviolet Long ultraviolet and visible

(0"2 -O-4 microns) (0 -4 0"7 microns)

Near infrared Far infrared(0-7 -- 1-4 microns) (1.4 100 microns)

Fig.l

100 He -Ne

90 - At R Rabbit

Human80Monkey (Rhesus)S70

S60

so N

%

30 IN-\

to

400 500 600 700 5O0 900 1000 1100 1200 1300 11.00 1500Wove length Inm|

Fig. 2Percentage of light incident on the cornea that is

absorbed in the retinal pigment epithelium and choroid

9999

99 9 /

I

99 I

I/ /

/I /90 /

I /

.• 70 / /.0//. so - --- ~

S30 -5"l, confdence I

30 951. confidence-limits

10 Ilii I

1 / ED 50

01 II

0011 2 5 !0 20 50 100 200 500 1000

Total ,ntrc-ocular energy (lJ)

Fig. 3Ophthalmoscopic retinal threshold damage probabilities

(2-switched neodymium laser. 30 micron image diameter).

99.99 -

99-9

99

90

Fluorescein

S70 Angiogrcphy

30 50550

-' 30

10 Lowest pthalmoscopy

detectable

Lowestdetectable

0/01 I I2 5 10 20 50 100 200 So0 1000

Total intro-oculor energyv(UJ)

Fig. 4Comparison of fluorescein angiography and

onhthalmoscopy as methods of detecting retinal damage.

S~8-1

DERIVATION OF SAFETY CODES, I U- USA EXPERIENCE

DAVID H. SLINEYLASER-MICROWAVE DIVISION

US ARMY ENVIMIENTAL HYGIENE AGENCY"ABERDEEN PROVING GROUND, MD 21010

SUMUMLRY

The first laser exposure limits in the USA date back to 1963. At first only two or three linits wereprovided. However, since 1972, a sliding scale of limits varying with exposure duration, wavelength, andPRF have been in use and are now standardized throughout the USA. The problems encountered in evolving

these limits and the complementary laser systen classification and field safety controls will bepLesented.

1. INTRODUCTION. Although the first safety codes for lasers date back to 1965, it was not until 1968that the first detailed code of practice was published in the USA. 1 This document, "A Guide ror UniformCodes or Regulation.s fox Laser Installations ", was developed by the Americas Conference of GovernmentalIndustrial nygienists 2 and for the most part was an adaptation of the first draft cf the US Army, TechnicalBulletin, TB MED 279.3 The exposure limits in this guide were first given widespread publicity when theyware recommended by the first International Laser Safety Conference which was held in Cincinnati, Ohio in1968. These exposure limits were based on retinal burn studies with only a few lasers - principally thenormal-pulse and q-switched ruby and neodymium lasers, and continuous white-light and argon-laser sources.For this reason the exposure limits were applied to only three exposure durations: 10 - 1000 ns, I us -

0.1 s, and "CV*. Different limits were then given for intrabeam viewing and for viewing extended sources.Thus, we had a set of six limits for visible and near-infrared lasers which could be adjusted slightly fordifferent wavelengths based upon the relative absorption of the laser radiation in the retina.

2. THE OPTICAL CAIN OF TOE EYE. At first we believed that different exposure limits should be appliedfor different pupil sizes.4 However, in 1967 an article by R.W. Gubisch appeared in the Journal of the

* Optical Society of America which modified this approach. 5 This article showed that although more energyentered the pupil when it was dilated, much of this additional energy wou.d serve tc enlarge the effectiveimage on the retina rather than add much to the retinal irraliance. This effect was later confirmed insome retinal burn studies performed by LTC Beatrice and his associates. 6 Therefore, in the ThresholdLimit Values publish.ed by the American Conference of Governmental Industrial Hygienists (ACGIH) in 1969and in the 1969 edition of the US Army's TB MED 279 on laser hazards, we provided only a single set ofintrabeam exposure limits for all pupil sizes. We still permitted a higher exposure limit for daylightconditions when the pupil was constricted and when we were dealing with extended sources. Fiqure I showsthe optical gain of the human eye for the intrabeam viewi'ig of a laser with the rilaxed normal eye.Notice that the optical gain varies very little with pup: 1 sizes between 3.-m and 7--mm and isapproximately 200,000. The peak irradiance in the retinil image t 0erefore largely unaffected by thisvariation in pupil size. We, therefore, conclude thzt the risk of in ury from intrabean viewing cf alaser is essentially the same whether we view the laser at night wit_ a 7-rm pupil or in daylight with a3-.c pupil, if the retinal injury thresho]i is dependent only upon retinal irradiance and not upon imagesize. In 1968, my colleagues and I assumed that there was no deperdence of retinal injury threshold withdiffering image size if the exposure duration was loss than 0.1 ms, during which there would beinsufficient time for the exposed retinal area to cool. It was well recognized at that tine that theretina could withstand an irradiance of 1 kWc=- 2 for several seconds if the image was of the order of 10pm to allow for cooling. At that time, many of us were inclined to believe that the aparent dependenceof q-switched (20 ns) exposures upon retinal image size was an experimental artifact. 7 10 We now knowthat this dependence is real. F~gure 2 shows that a spot-size dependence is observed for a wide varietyof exposure durations. Figure 3 also demonstrates this dependence.

3. STEP FUNCTIONS. On& of the chief complaints leveled at our simplified set of exposure limits was thepresence of step functions. By this, I mean that the levels jumped a factor of ten at one point oranotheri for instance at 1 microsecond. At first this was no problem. But then lasers began to appearwhich had a pulse duration of 1 microsecond. Which level applied: The value of 10-7 J3cm- 2

based onq-switched exposure data or 1076 J-cm"2 based on normal-pulse data? This of course, was not the onlyproblem. The CW laser exposure criteria of 1 - 10 uW*cm 2 did nact allow for a m¢mentary exposure but wasbased on the assumption of certain long-term effects. These ACGIH limits which were adopted by the US Armyand US Nawy were not adopted by the US Air Force. Scientists within the US Air Force felt theaforementioned spot-size dependence was real, and furthermore felt that no individual would continuouslystare into a laser for more than a second. The US Air Force exposure limits1 1 in 1969 were therefore muchgreater than the %rmy/Navy/ACGIH limits as is shown in Figure 3. Aside fron the exposure limits for theeye, these standards recommended most of the same safety measures and medical surveillance. The skinexposure limits were the same. Since the Armed Services in the United States had sponsored most of thebiologic rasearch and accounted for much of the laser development in the USA during the 1960%, their

standards were gener-lly followed by industry to a large extent. But by 1969, it became clear that anational consensus standard on laser safety was desirable. The ACGIH standard, although national inscope, was developed and controlled by governmental industrial health personnel, such as myself, andindustrial personnel could not play a role.

4. 2WE ANSI-Zt36 STAMDRO DEVELOPMENT. In 1969 an effort was initiated by the American NationalStandards Institute (ANSI) at the request of the US Department of Labor to develop a consensus standardfor the "Safe Use of lasers and Masers," later redefined as 'Safe Use of Lasers-Standard Z136." Thetelephone group became the sponsoring activity, and Mr. George M. wilke.zinq of Bell Laboratories wasdesignated chairman. When work got underway with subcommittees formed in 1969, the hope was expressedthat a final standard could be completed in 1 year if all subcommittees worked diligently. The

C 8-2

A

WO

SLa

F Thoreticol

Z

0F l0 Comsthone

I I hqokIcI x SeeI

"I0 2 3 4 5 6 7 8PUPIL DIAMETER (mm)

IREJ 1. Influence of pupil size on the optical gain or magnification factcr of corneal-to-retina irradiancefor a point source viewed by the normal human eye. Theoretical curve was obtained using the 'i',yformula for peak retinal irradiance (Sliney. 197:). A second curve shows the optical gain for aconstant retinal image aiameter of 10 ;j. The final curve is believed to more accurately representthe actual optical gain and was derived by multiplying the theoretical values by the Strehl ratiosreported by Gubisch (1967). The Strehl ratios could be high (Sliney and Freasier, 1973) .21

8.3

ILLI.- OM 62$U

10 %.00 100 1 07 0 M

.. • •RiETINAL IMAGE DIMEE (Im

Si tin• 1=* Size. 1,,7 Also frcomoarlson, rabbit data fra 1LO s exposureIshon

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84

S/ .LUJ

w10

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z

-ANSI Z-136 (I.U_!.• I____.

Z 6710

EXPOSURE DURATION (s)

FIGUIM 3. Selected data from many e~tperiment$ which attempted to determ.ne the laser retinal injurythreshold in the rhesus monkey for the minimal ima•ge condition. Plotted for comparison arefour protection standards applicable to intrabeam viewing (minimal image) condition. Biologicaldata points represent ED30 values of Ham et al. (1970a) for He.l () Dunsk ad ap 191for krypton (0). Bresnick et• al. (1970) for argon (A); Vassiliadis et al. (1971) for doubledneodymium, 530 =• [I, and for ruby (0)1; Vassiliadis et al. (1969) for argon, 514.5 nm (0);Lappin (1970: for He-Ne, 632.8 =• (7)o Naidoff and Sliney (1973) for welding are point source M;}Skeen et al. (1972a) for neodymium•, 1064 nm M;} and Skeen et al. (1972b) for argon, 514.5 nm(-)o21

USF7 &-.1

experienced staff at the headquarters of ANSI in New York doubted that the standard could be prepared inless than 3 years. A general consensus was not achieved, however, until a cormmittee vote on the lastofficial draft, dated February 29, 1972. In May 1972, the ACGIH proposed a revision of their TLVts forlaser radiation, which incorporated essentially all of the ANSI-Z136 Maximum Permissible Exposures(MPE's). The ANSI standard was quite complex and because most of the ballots on the February 29th draftiudicated a desire for editorial changes for clarity, the final draft was considerably revised and wasissued on November 23, 1972 as a final draft for submission to the ANSI Board of Standards Review. Thedocument was approved on April 26, 1973 and was issued in October 1973.12 The military services, andother Federal and state agencies that make use of laser protection standards, adopted most of the newprotection standards promulgated by ANSI and ACGIB.

5. FO.MULATION OF LASER PROTECTION STAWDARD EXPOSURE LLMITS.

a. The greatest departure in format made by the ANSI standard from previous standards is the lack of"step functions" to express the MPE levels. Previous protection stardards all had values expressed asradiant exposure (J-m 2) or irradiance (W-cm- 2 ) for a specific range of pulse durations. vith the adventof lasers having any pulse duration, it was necessary to provide a sliding scale without sudden "steps" atspecific pulse durations. Indeed, such an approach permitted a closer approximation of actual biologicinjury thresholds with safety factor added.

b. To establish a rationale for developing permissible exposure levels from biologic data required acareful analysis of the physical and biologic variables influencing the spread of the laboratory biologicdata, the variables influencing the potential for injury in individuals exposed to laser radiation, theincrease in severity of injury for supra-threshold exposure doses, and the ret:ersibility of injury.Pdditionally, the accuracy of instruments available for radiometric measurements and the desire forsimplicity in expressing the levels have influenced the protection standard levels.

c. It was difficult to prorerly weight these many factors. Interestingly enough, I took a poll ofseveral specialists who had involved in the development of the protection standard levels showing thatalthough almost all of the specialists agreed to a certain set of %VPE's or TLV's, they had a wide range ofdifferent rationales.

d. There was never any serious discussion of having separate military and ci-ilian protectionstandards, since it was always agreed that a threshold of injury existed. No be.efit-vs-risk analysisapplied to the setting of the standard values. Benefit v. risk analysis was to be applied in the fieldevaluation of lasers.

6. LASER HAZARD CLASSIFICATION AND CO.TROL MEASURES.

a. The WNSI-Z136 standard contains a well-developed and formalized scheme of classifying lasers basedupon the laser's degree of hazard. This scheme evolved from previous standards and guidelines 1 3 andpernits rapid hazard evaluation. Specific control measures and medical surveillance requirements varydepending upon classification.

b. In the final analysis, the specifications that define the hazard classes will be used more oftenthan the protection standards (CMPE's) themselves. Five classes are defined: Class 1 Exempt Lasers arethose lasers incapable of producing a hazardous exposure condition; such lasers are unusual and generallylimited to laser diodes. Class II Low Po..er Lasers are visible lasers (usually He-Ile) with an outputpower below 1 eW which are not hazardous unless an individual looks directly into the beam against hisnatural aversion response (i.e., longer than about 0.25 sec). Class III Mediun Power Lasers requireprecautions to limit intrabeam viewing of the direct beam or a specularly reflected beam. The laser doesnot present a fire hazard, a skin hazard, or diffuse reflection hazard. In contrast, a Class IV HighPower Laser does present a fire and skin hazard 3nd/or a diffuse-reflection hazard, and very stringentcontrol measures are required. Class V Enclosed Lasers, as the name implies, are those within aninterlocked enclosure such that emitted laser radiation from the enclosure is not hazardous.

7. A SUMARY OF THE ANSI AND ACGIH EXPOSURE LIMITS. In this brief lecture I cannot go into much detailabout all of the exposure limits given in Tables I-IV. These protection standards are intended forexposure to laser radiation under conditions to which nearly all personnel may be exposed without adverseeffects. The values were intended to be used as guides in the control of exposures and should not beregarded as fine lines between safe and dangerous levels. They are based on the best availableinformation from experimental studies performed up to 1972.

8. LL1.ITING APERTURES.

a. One of the problems in developing exposure limits for any standard is the specification of thelimiting aperture over which the level must be measured or calculated. For the skin, where no focusingtakes place, one would like to have as small an aperture as possible. Unfortunately, the smaller theaperture, the more sensitive an iastrument must be, the greater the inaccuracy will be due to calibrationproblems and the more difficult the calculations may be. We felt that a I-em aperture was about thesmallest practical size to consider. For continuous exposure conditions, heat flow and scattering in theskin would tend to eliminate any adverse effects or "hot spots" which were smaller than 1-ms. The samearguments hold for exposure of the curnea and conjunctiva to infrared radiation of wavelengths greaterthan 1.4 um. Furthermore, atmospherically induced "hotspots" and mode structure in multimode lasersseldom account for localized beam irradiances which are limited to areas less than 1-m in diameter.Another problem appears at wavelengths greater than 0.1 um. At these far-infrared wavelengths theaperture size of I-=i begins to create significcant diffraction c :ects and calibration becomes a problem.However, "hotspots" must, by arguments of physical optics, be generally larger than at shorterwavelengths. For this reason we chose a 1-cm or 11-mn (which has a I cm 2 area) aperture for wavelengthsgreater than 0.1 mi.

8.6

b. In the retinal hazard region of the spectrum, which extends from approximately 400 run to 1400 M,the aperture over which the incident radiation can be averaged is the pupil of the eye, if the exposurelimits refer to the eye. A pupil size of 7-tin was finally decided upon, although not without a great dealof debate. I am sorry to report that the 7-iun aperture was influence more by political than scientificconsiderations. If you recall, I pointed out earlier in Figure 1 that the optical gain of the eye waslargely unaffected by pupil size. From this viewpoint a pupil size of 3 vu would appear more reasonable.without going into detail I will simply say that certain laser products faired better if I milliwatt intoa 7-run aperture rather than 0.2 MW into a 3-ma aperture were chosen as the upper limit of Class II - LowPower Lasers. It is only fair to point out, however, th&t either approach lacks support from actualretinal burn data. Because of the spot-size dependence of retinal injury thresholds , the 7-gm limit isnot far from being the worst-case condition except for exposure durations of approximately a microsecond,"where the spot size dependence of injury thresholds now appears to be the least. Scae experts in the USAstill maintain that a 5-am aperture is really the worst-case conditio:.. In any case, the present exposurelimits have a factor built into them which accounts for the expected variation of the wcrst-case aperturedepending upon the exposure duration.

9. REPETITIIELY PULSED LASER EXPOSURE. The present exposure limits in the USA for repetitively pulsedlasers are based upon very limited data. I think that I can truthfully say that no one in our country hascome up with a satisfactory explanation for the variation of the retinal injury threshold with cumulativeexposures to short-pulses. By short-pulses in this regard, I mean pulse durations less than 0.1 as. Onewould expect an additivity based upon the duration between exposures. Specifically if the pulse intervalware less than 0.1 ms during which very little heat flow would take place, the exposures should add verynicely. They do not. Furthermore, if the pulses are separated by a period of time sufficient for thetemperature to decrease to ambient, an additive effect is still noted for q-switchd pulses. Until theadditive mechanism is understood the present exposure limits =est lie treated with great care. Figure 4shows the correction factor that applies to pulse trains where each pulse lasts for less than 10 um.

T.4BLE L Proteftion Stsndards for Typical Lasr

Protection standard forintrabeam rkicwng

Type of laser PRF W'a.ciength Expo Ane duration by the cry

Smngle.pulse ruby law Single pilse 444 , n.n I nsc-IS psec 5 X 10" J-cm-I/puhdSmnZt-pulse nrldnmum S:nmh- pul., lo.o nm I owe-I(S) i 5 X 10" J-cm-t /p"seC-%%" argon lasers CW 4P1.t fnm. ;44 "rnm 0 235 Svc 2 5 mnW*cm-'C%" arsgon lAses %U" 48, ni. A14 n nm 4-8 hr 1 .-WCM"CV%" hrl,um-nron la;ers (for (%V" 8 nr, ') 3 s.c 2.5 MW-cMs1

al:gneist. cic ) 4-8 hrs I pW-cmV-'Erhbum lasWr Single pul., 1I"4 am I nsec-l ec I J-Cm-'/pulsc(%V nrodtmum Y%'G l ~t CV I-,A nm WI-) we-8 hr 0.3 mWV-CM'CW cxbon-diomsd las-f C" 20 i, 5 m I0 s•-8 tr 0 h j .- 21

511.0 t i T .. j I a in i li i I In 1i 1i 1 1 " I

U)-J

f 0.1

U)0

-J

0.10 100 0000 10.000

PULSE REPETJTION.FREOUENCY (142)

FIGURE 4. Correction factor (Cp) for repetively pulsed lasers having pulse durations less than 10-5 sec.The protection standard for a single pulse of the pulse train is multiplied by the abovecorrection factor. Cp for a PRF greater than 1000 Hz is 0.06. Experimental data: 0, argon M,noodyiu. From Skeen et al. (1372).

8-7

10. THE5 "SAFET FACTOR".

a. The margin we introduced to account for experimental error and errors in applying the exposure* !limits was very difficult to arrive at. To illustrate our problem consider the interpretation of one

SIIthreshold point from a research study. This threshold is normilly the result of considering a probitanalysis of many data points.

b. Many laser retinal burn studies have tried to simulate the °rct-case" exposure fr asafetystandpoint, i.e., the relaxed normal eye exposed to a collimated beam - the minimal 10- to 20-1 retinalimage. Consider the problems of determining the injury threshold for a 1-sec exposure of a minimalretinal image in a monkey from a CW visible iaser. Obviously, measurement errors are introduced durinqthe measurwsent of the laser power entering the eye and of the pulse shape. If the retinal injurythreshold is strongly dependent on image size, a -mall error of +0.25 diopter in refracting the monkeycould result in a far-frcx-minimal image size of 35 - 45 u. Likewise, such an error could be introducedif the monkey's acommodation drifted +0.25 diopter while under cyclopleDia. During a 1-sec exposure,even s-hallow breathing and blood circulation of the anesthetized monkey would cause image wander

( significant in comparison to the image size. It is well known that the eyedrops used as a mydriatic andcycloplegic create a noticeable corneal haze in ovexdo!,e, but what if lesser doses produce a corneal hazenot readily evident to the experimenter? Thi;s introduction ,:f additional scattering of the light couldgreatly reduce the laser power delivered to the central image, and a large percentage of the power couldbI delivered outside of the image unnoticed by the experimenter. N•onuniform retinal pigmentation andother anatomical factors will further spread the data.

c. If an experimenter made a great effort to place retinal lesions only at sites he judged to be ofthe same uniform pigmentation, one would expect a relatively steep ED CURJE to be the result. On theother hand, if he completely ignored this factor, the ED CURVE would have a shallower slope. Minimalretinal lesions placed in the vicinity of the optic disc could require an exposure dose 50-percent greatertIan that required for the same type of lesion in the center of the macula1 5 Still another parameterwhich may affect the retinal burn threshold, body terperaturel , is often overlooked and not controlledduring experiments.

d. It is interesting, considering the source of errors, that occasionally an experimental data pointprobably does approach or even achieve the "worst-case." Figure 5 shows a hypothetical guess at theposition and shape of the actual best-case, error-free experimental curve vs. a possible error-riddenexperimental curv&. It is therefore not at all surprising that the standard deviation for retinal injurythresholds for large retinal images (e.g., 500 ý.) is far smaller, and hence the slopes of aba probabilitycurves are much steeper 17. The more errors present, the less steep the slope of the curve. Hence if onechooses a sufficiently small probability ordinate poDnt for a "safety" level, the standard writer is stillsafe.

999-

99.0

EXOS C m

s750

S50

S25

10,

a:IL

01

2 4 10 20 40 too0200EXPOSURE - m"W

T'.oRE 5. Hypvothetic.31 error-free exn.rinonral curve vs. an error-ridden curve -or An exnerinentaldeternination eý laner retinal burn threshol,1 'or the nini.al i.ane size. Ri-tht-handcurve u-uld. represent lata noints where the .inirmal image size was seldom achievea.

8-8

11. SPECTRAL DEPENDENCE OF EXPOSURE LIMITS. Injury thresholds for both the cornea and the retina varyconsiderably with wavelength. We must consider how detailed we wish to track the actual injury thresholdvariation with wavelength. Normaliy the solution to these problems is a compromise: the protectionstandards are adjusted for different wavelengths but in a more simplistic manner than the actualbiological data could permit. Figure 6 provides the product of the relative spectral transmittance of theocular media and retina &bsorption, which is an indication of the relative spectral effectiveness ofdifferent wavelengths in causing retinal injury.1a 19 However, these curves do not show the relativehazard to the lens of th2 eye in the near infrared. This is also plotted in Figure 6 for comparison withthe spectral correction factor used in the ANSI-Z-136 standard.

F lun 6. Normalized plot of reciprocal oC theretinal absorption of optical radiationincident on the cornea based on

10D the data of Geeraets and berry (1968)and Boettner (1967). The relativespectral correction factor (CA)used in the ANSI Z-136 standardsis shown for corparison.

0

Z0I-

UwM 10 T

0

0 CA

0

z

1.0

400 600 800 1000 1200 14C:WAVELENGTH- nm

12. TYPES OF STANDARDS. EXposure criteria may be reflected in two general categories of standards:oc-upational health and safety standards and equipment performanc5 etandards. Within the US Government,occupational exposure standards are enforced by the occupational Safety and Health Administration (OSHA)of the US Department of Labor. Federal product per'ormance standards are enforced by- the Bureau ofRadiological Health (BRH) of the US Depaxtment of Health, Education, and Welfare. Federal standards foroccupational exposure to nonionizing radiation have existed for microwave radiation and visible cw laserradiation since 1971. This year proposals for an ultraviolet radiation standard4 and a laser radiationstandard are under consideration within the Department of labor. A proposed performance standard forlaser products has been published in the Federal Register and could take effect as early as next year.

13. FUTURE OUTLOOK.

a. The principal protection standards for laser radiation are not likely to change for some time.However, the protection standards for repetitive exposures, long exposure durations (greater than a fewminutes), and for wavelengths outside of the visible bond (i.e., infrared and ultraviolet radiation) werebased upon a considerable amount of extrapolation. 1 7 One can expect, therefore, that progress may beforthcoming in some of these areas of biologic research. Several groups are exploring the retinal injury

18920thresholds for groups of short pulses. . Little progress has been made recently in determing injurythresholds from ultraviolet and infrared laser radiation, with the notable exception of several US AirForce studies. One theory has been proposed for the action spectrum of UV-induced cataracts, but this isyet to be proven.tl Preliminary data for retinal burn thresholds for :0-psec (picosecond) mode-locked

"= 8-9

laser exposures obtainod by Ham and co-workers shoied that thresholds were reduced by a factor of at least10 when compared with 30 ns (nanosecond) exposu:es tc. a mintLial image area of the retina. 2 0

b. Recent studies of long-term exposure of the retina to visible light have shown that we canprobably increase the long-term exposure limits of laser wavelengths in the 500-900 =m spectral region.This conclusion Is basedon the action spectrum of the photic effect which manifests itself for exposuresgreater than I second.

14. FIELD MILITARY CONSIDERATIONS.

a. By ccntrast to industrial safety codes, military laser safety codes consider only one additionaltopic-field laser operations. The policy for use of tactical laser rangefinders and designators requiredseveral key decisions. The decision had to be made whether a "safe" viewing distance could be determinedfor a specific laser device. The questions on the effects of atmospheric scintillation on the bean andthe effects of viewing optics influenced this decision. Finally, the question had to be answered whetherabsolute safety could be assured on any range if specularly reflectinq surfaces were present. If"absolutely safe operations were not possible, a certain acceptable risk-level would have to beestablished.

b. The cafety implications of atmospheric scintillation of laser beams have long been recognized.With the testing of military lasers in the field, the "hotspots" created by atmospheric turbulence werelooked upon as an uncertain variable in determining a laser 3 hazardous range 22. In the past 10 --ears,

- sesiral studies have been performed largely with the He-Ne lasers, in an effort to quantify L.:;effect. 2 3 2 6 These studies resulted in statistical probabilities of finding an irradiance "hotspot" acertain factor above the mean. Deitz developed a nomogram (Figure 7) for such a purpose. 2 3 There hasbeen some debate however, as to how useful such statistics are and whether they give the complete answerfor safety a..alysis.

_T- -

crA a.05/km•-:.4 Cnt 5 x to-7 -1

.E 0" jim2

, 69431

I%0

t ,: . .

2 3 4 5 6 7 8 9 0RANGE (kin)---A9

FIWRE 7. :.ye safety no.amraph of Dietz for C , the refractive index structure coefficient, typical ofstrong turbulence. The atrospheric attenuation coefficient, a, is typical of very clearseeing conditions. The value ET was thtM "safe" level of expoaure and E was the outnut energyaf the laser in joules. From reference 21 with permission of Applied Opics. The bean spreadof the laser bean is expressed as a isilid angle. (I.

In numerous tests of field lasers by myself and my associates at the US Army Environmental Hygiene Agencyover the past decade, we have found that the probability of occurrance of significant "hotspots" seems tobe less than reported in the other studies. Our explanation has been that the other studies did not takeinto account the beam spreading which is always present during periods of strong turbulence. Thisspreading of the beam reduces the average beam irradiance such that the excursions of localized beamirradiance above the average are not as serious as they appear to be at first glance. 26 8

c. Most studies were performed by atmopheric physicists who were looking only at the irradianceprofile of the laser beam, i.e., the beam "cross section". They did not look at the projected radiance ofthe laser. The latter parameter is, however, of significance from an eye-safety standpoint. The raaianceor "brightness" of an extended light source determines the irradiance falling on the retina.2 7 However,the parallel rays of a point source, are always imaged on the retina as a minimal image. A source such asa laser is effectively a point source when the intrinsic divergence of the light rays entering the pupilis less than 0.3 mrad. %'hen "turbulons" in the atmosphere tend to focus the collimated rdys in a laserbeam, the focused rays can have a divergence greater than 0.3 mrad and the resulting retinal image isincreased. The safety question that must be answered where the eye is located in a "hot spot" is whetherthe retinal irradiance will be increased over that occurring during quiet atmospheric conditions.

d. For an extended source we can apply The Law of Conservation of Padiance. A glass lens (or aturbulon) or a telescope cannot increase the source raiance, and therefore cannot increase the retinalirradiance of a searchlight or other light source that is already resolved by the eye. If, however, thesource cannot be resolved by the unaided eye or by a telescope, then a turbulon can increase the apparentorightness of the laser. At the US Army Environmental Hygiene Agency we conducted an evaluation ofphotographic -,-.ages of a laser over several atmospheric path lengths. This effect on the extended imageof vore-or-less constant irradiance is shown in the photographs of Figure 8. Using a beam splitter andirradiance monitor in front of the camera, we were able to correlate beam irradiance with each image. Themicrodensitometer scans also permitted a careful analysis of the irradiance profile on the film.

//

t I

-

"".... F,(2 .4,."iinI maesafecedb

S- - -": :-

"P2-- a shrctublne

i, •8-I I

e. Based on such experiments we now feel that turbulence does rot so siqnificantly add to the laserhazard as was belicved in the past. But there is another factor that has not yet been mentioned. So farwe have tacitly assumed that laser-induced retinal injury is only dependent upon retinal irradiance. Thisis not the case for many CW or short-pulse laser exposures; the injury threshold irradiance decreases forincreasing image size. 6 18 This greatly complicates the determination of the increased retinal hazard dueto scintillations. The exposure from a CW laser of course becomes averaged over a number ofscintallations, and we disregard scintillation for these lasers.

f. When we finally add all of the probabilities we realize the small chance of an accident. Considerfirst, that au individual is looking into a pulsed laser; second, that his eye is focused at infinity andhis fovea directed at the laser; third, that his eye is within a significant hotspot; fourth, that the

Sweather provides a good visual range; and furthermore, that his retina is more sensitive (more absorbing)than average; the chance is vanishingly small. The added risk of someone being within the beam with atelescope or binocular is probably greater. For this reason we normally insist in the US Army that thebeam be torminated by a backstop such as a hill within a controlled area 2 9 unless it is pointed skyward.Fortunately people in aircraft being unable to stabilize a binocular do not use them and atmospheric

turbulence is far less for a ground-to-air path. In the latter ca.se, we calculate the hazaruous range andwe feel that it has meaning.

g. The control of specular reflections becomes the next remaining problem. Our approach has been toeliminate all flat surfaces of glass that may be exposed in the target area. If this is not possible, wetry to operat-. it. a clearing in the woods or in a valley, where no unprotected persounel can see the laseror the target. At first, this may sound very difficult to you, but I can assure you that we have neverencountered a test or training exercise where we could not fullfil the safety restrictions.

h. The last major problem that one must face in establishing a safety program for military lasers isassuring that the beam is pointed into a controlled target area. This in largely a problem of beamalignment with the laser sighting telescope. Maintenance personnel must be aware of the importance ofassuring this. On the range, a range safety officer must be sutre that targets are not fired at within acertain buffer angle of 2-10 nils from occupied areas. we establish the buffer angle based upon tests ofpointing accuracy of the beam and stability of the laser mount.

15. CONCLUSION. Although there is still much to be learned ahout the biologic effects of certain typesof laser exposures, most of the present standards will probably remain intact for a considerable time tocome. This year the XNSI Committee Oet again with the intention of modifying some of the Iona-termexposure limits, but no action has yet to be comoleted.

TABLE II. frotection Standards for Ocular Ezpoeure Intrabeam Viewing of Lase Beam Single Ekpoeures

Spectral Wavelength Exposure time Defiingreion (nm) (9) Protectioe standard aperture (,m)

IUV-C 200-280 1 .onc to 3 X 10' sec 3 mJ.cm- IUV-B 280-302 msec to 3 X 101sec 3 mJ-cm-' I

303 1aec to 3 X 10'sec 4 mJ.cm-' I304 1nuc to 3 X 10'sec 6 mJ.cm7 I305 1 ecto 3 X 10 sec :0 mJ-cm'- I306 Msec to 3 X 104sec 16 mJ-cm- I307 1nsec to 3 X 10' wc 25 mJ-cmr-308 maec to 3 X 101'sce 40 mJ-cm-2309 1nec to 3 X 104 sec 63 mJ.cm"n310 1 nscto 3 X 104 we 100 mJ-cm-' I311 Imsec to 3 X 104 se 160 mJ.cm-2 I312 msec to 3 X 101 scc 250 mJ-cm- 3313 1msecto 3 X 10sec 400mJ-cm- 3

314 1 msecto 3 X 101see 630 mJ.cm-315 1sec to 3 X 10 $'c !.0JaM-- i

UV.A 315-400 I nsec-t0 swe 0.56*'t J-cmn - 3315-400 0 sec-10%s$c 1.0J-cm-2 I135-400 10' to 3 X 101 sec i.0 mW-cm"4

L.ight 4M0-7M) 1 mi-18 pd-: 0.5 iJ-Ccm- 7441-7M0 18 ,sec-10 sec 13 .8t/] mJ-cm-' 6 74100-700 10 sec-310 sec 10 mJ-cm-2 7400-700 101 to 3 X 10' sec i aW-cm- 7

IR-A 700-3060 1 nsec-18 pvc 0.5Cat J-cm-n 7700-1060 18 $sec-10 sec [1.8/l€/s]Ca mJ.cm- 4 7700-1060 30 se-l0O sWc iOCA mJ.cm-' 7

1060-1400 1 nsec-100 asec 5 PJ.cn-m 71060-1400 100 jec-10 see [91/-I] mJ-cm-' 71060-1400 10 sec-i 00 sec 50 mj-cm- 7700-800 100-f104/Ca] sec iOCA mJ-cm-* 7700-800 [I0'/e] to 3 X 104 sec CA.C* tW-cfc-' 7800-1060 100to 3 X 0'sec 0.A CA mW-cm-2 7

1060-1400 100 to 3 X 101 sec 0.5 mW-cm-2 7IR-B and -C 1400-10 3-I00 nsec 10 mJ-cm-2 3,10,

1400-10' 100 nsec-10 sec 0.560/t J-cm-n I,301400-10' 10 to 3 X 10' sec 0.1 W-Cm-2 1,10

""CA - (A.700/224); Ca = (x-699).

'Th.me values are graphed in Figs. 12 and 17.I mm for 1400-10' nm: 10 mm for 106-104 nm.

REFERENCES

1. Sliney, D.H., The development of laser safety criteria - Biological considerations, in: LaserApplicitions in !Iedicine and Biology (M.L. Wolbarsht, ed.), Vol. I, pp.163-238, 1971.

2. ACGIH, A Guide for Uniform Industrial Hygiene Codes or Requlations for Laser Inztallations. TheAmerican Conference of Governmental Industrial Hygienists, Cincinnati, (March 1968).

4-.

3. US Department of the Army and US Department of the Navy, :Zontrol of Hazards to Health from LaserRadiation, TB MED 279, NAVMED, P-5052-35 (February 24, 19o9).

4. Sliney, D.11., and Palmisano, W. A.; The Evaluation of Laser Hazards. Presented before the May 1967meeting of the A-Lerican Industrial Hygiene Association, Chicaao, in Am Indust Hyg. Assn. J, 29:425-431,(19h8).

5. Gubisch, R.W., Optical performance of the human e,'e, J. Opt. Soc. Am. 57:407, (1967).

6. Beatrice, E.S., and Fris-1, G.D., Retinal laser daumage thresholds as a function of imaqe diameter,Arch. Environ Health, 27:322-'26 (1973).

7. Ham, W.T. Jr., Geeraets, W.J., Williavms, R.C., Guerry, D., and Mueller, H.A., Laser radiationprotocticn, in: Proceedings of the First International Congress of Radiation Protection, pp. 933-943,Pergamon Press, New York, (19681.

9. Sliney, D.H., Evaluating health hazards from military lasers, J. Am. Med. Assn. 214:1047 (1970).

9. Mainster, Y.A., White, T.J., Tips, J.H. and Wilson, P.W., Retinal-termperature increases prcduced byintense light sources, J. Opt. Soc. Am 60:264 (1970).

10. Clarke, A."., "Ocular hazards from lasers and other optica' sources," Crit Rev Environ Control,1(3):307-309 (November 1970).

11. Carpenter, J.A., Le =miller, O.J. and Ttedici, T.J., US Air Force permissible exposure levels forlaser irradiation, Arch Environ Health, 20:171-176 (February 1970).

12. A-.merican Naticnal Standards Institute, Z-136.1 Standard. The Safe Use of Lasers, New York, (1973).

13. Sliney, 9.F.: Evaluating Hazards - and Controlling Them, Laser Focus, 5(15):39-42 (August 1969).

14. Van Buskirk, C., NWblbarsht, M.L., and Stecher, K., The nonnervou- causes of normal physiologictremor, Neurology 16:217 (1966).

15. Lapp'n, P.*-., and Coogan, P.S., Relative sensitivity of various areas of the retina to laserradiation, Arch. Ophthalnol. b4:350 (1970).

16. ward, B., and Bruce, :.R., Role of body temperature in the def~nition of retinal burn threshold,Invest. Oohtha.nol. 10:955 (1971).

17. Ham, X.T., Jr., Geeracts, w.J., -ueller, H.A., Williams, R.C., Clarke, A.M., and Cleary, S.F.,Retinal burn thresholds for the helium-neo,, laser in the rhesus monkey, ,rch. Ophthalmol. 84:797 (1970).

18. Baettner, E.A., and :€nter, J.R., "Transmission of the ocular media," Invest Ophthal, 1:776-793(1962) (AD 282100); see also Boettner, -.A., Suectral transmission of the Eye, Final Report AF 41(609)-Z996 iJuly 1967) (AD 663246).

19. 3eeraets, W.i., and Berry, E.R., "Ocular spectral characterictics as rel~ted to hazards from lasersand other light sources, "S.-er. -.. ophthal 66:15-20 (July 1968).

20. Ham, W.1., M, ueller, H.A., Goldman, A.!., Newman, B.E., Holland, L.M., and Kuwabara, T., Ocularhazards from picosecond pulses .)f ::d:YAG laser radiation, Science, 185:362-363 (1974).

21. Sliney, D.H., and Freasier, B.C., The evaluation of optical radiation hazards, Ap.l. Opt. 12:1-24(1973).

22. Sliney, D.H., Comments on atmospheric turbulence, in Proceedings of the First Conference on LaserSafety, pp.86-87, •:rando, FL, (M1a-, 1966), Vartin-MYrrietta, Orlando.

23. Deitz, P.H., Probability Analysis of coular damage due to laser radiation through the atrosphere,Appl. Cot., 8(2):371-375 (1969).

24. Deitz, P.H., Safety considerations in outdoor applications, Laser Focus, 6(6):4(.-43 (June 1970).

25. Dabberdt, ".F., Slant-path scintaillation in the planetary boundary layer, Appl. Opt. 12(7):15'.6-1548(1973).

26. Yura, H.T., Atrospheric turbulence induced laser beam spread, Appl.Opt., 10(12):2771-2773 (1971).

27. Sliney, D.11., an! Freasier, B.C., Evaluation of optical radiation, Appl. Opt., 12(l):1-24 (1973).

A •8-13

28. Marshall, J., Sliney, D.H., and HillEnkamp, F., Damage mechanism for pulsed laser induce,' retinalinjury, (to be published).

29. US Departnent of the Army, Control of Hatards to Healtn rrom Laser Radiation, TB MED 279, Washington,:DC, US Government Printing Office (1974).

TABLE IM. Protection Standards for Skin Exposure to a Laser Beam

spt ti .d Exposure time Protection,Wavelength (1) (scc) standat d

tIV 2t0-40M nmn 10- to 3 X 104 (Same as 'Fable I)lai.it and IR-A 400--1400 nm 10-' to 10-7 2 X 10-2 J-cm-2

Li.aght mid IR-A 4M0-1400 nm 10 -7 to 10 1.1 -1 J-cm-1 a

iheht .antd IR-A 400-1400 nm 10 to 3 X 10' 0.2 W-cm-2

IR-B and -- 1.4 9m-I mm 109 to 3 X 10' Same as Table II1. Im I mm 10SmIsTbe1

TABLE 11. Protection Stantdard- for Laser Radiation Exposure of the Eye Vieing F-tended Sources and DiffuseReflections 400-1400 tin

%N .t,. nitt l In di:,uht, or rd(fa-it ,c,%Is r," Rai;. n - " ,,r :nt'"rdt'-d radianc:

f11M IEm o'-;i. otn t : IPl fml r -" p, rfi zt .. from , ".t, . d -o-ur,-

.. 1-4,1 i nit c to I .. i i. cc (Use vAluts in Table 11)

4-ut-711 ! nicc-I -cc IlOr'? :J cM 112. J-cm---sr-:

l.i-Ill st'e" 2O'r J -*m- 211 J-cm-"-sr--I'll to I "V I0," sc- 2r X 10-1- Wcm-: 2 X WI W W-cm--sr"-

7t1-lfto.1 I nst-c-I It w' I417CA? - J.Cm2

111CR.:, J-L1-±-rr-71'i-IOIi Il-11%c sec 21r7CA .I-Cm-' 21)CA J-tcm-tsr-'

Ii') to (lip' C01 s'c 201Cr. J-cm 20CA J-cm-"'sr-I7(1-H1ll, (Ii r'c ir 2rC.C V-Cm 1.2C.CS W-cm -sr'HlJ-I It I If?() to 5 3' S $c YI.2C.I W-cm-' 0.2C.4 W-ctm"--sr-'

II$,I-I4,J1 I nsic-IO %cc c r5,11J-Vm: I' 50-t t J-Jcm-!-sr-"

1fi.i-144$ I1-00 see I Nir J-cm-: 100 J-cm--.sr-I111*€114111),1 to S .t l -' Sec r W -cm-' 1.0 W -. m-:-sr-'14101-I06 If nsec to i / I(1 sre- (Use values in Tabie 11)

III$

9-1

DERIVATION OF SAFETY CODES II - UK EXPERIENCE

R.G. BorlandRoyal Air Force Institute of Aviation Medicine,

Farnborough, Hampshire, United Kingdom.

SUMmARY

The initial approach to laser safety by the United Kingdom was based on limited experimental dataa.d so tended to be over-cautious, but recent studies, both in the UK and USA, have been related to thepractical situation of ocul'i, irradiation by parallel beams and has suggested that the retinal radiantexposure for damage increases with decreasing image size. This study suggested that a considerable relax-ation of the British Standards Institute (BSI) recommendations published in 1972 was possible. The inter-national use of laser equipment has emphasised the need for a unified approach and the United Kingdomevaluated the exposire levels recommended by the Amerizan Conference of Governmental Industrial Hygienists(ACGIH) 1973 and incorporated in the American National Standards Institute (ANSI) Report Z 136. As aresult the BSI has recommended the use of ACGIH exposure levels. Similar exposure levels are being adoptedby many European countries, and have been recomended for use within NATO. Yt is hoped that it will bepossirle to update the ANSI recommendations on an international basis in the future.

A code of pra-tice should be based not only on realistic exposure levels for damage, but on a systemcf classificdtion whicn can identify lasers according to thcir hazard potential. For some military appli-c ations 'safe' viewing distances based on the pr-sent standards may prove operationally unworkable, and amore realistic approach to laser safety may be required.

iTRODUCTION

The nistory of Code of Practice and safe exposure levels within the United Kingdom can ne divided intothree main periods. The first codc of practice for laser workers was published in 1965 by the then Ministryof Technology wnc at the time represented the largest single user o4 aser equip.ent in the United Kingdom.in !968 the Britisn Standards lnstit~tion issued a draft Code of Praýtice based in the earlier Ministry ofTechnology code which after modification and updating was issued as Lritish Stan•)ard 4803: 1972 (Ref. 1).During tnis same period several other organisaton and agencies published guides to their workers basedon either tne:r own interpretation of the limited research data available or on the early American lasersafety ;ides, notably that published by the American Conference of Governmental Industrial Hygienists in19C5. in retrospect it is perhaps unfortunate that the British Standard c¢de was being written during theperiod _968-1970, for wnile being able to acknowledge the work of Vassiliadis et al (Ref. 2) we did notreflect the more recent work of this author and others which has subsequently led to the better dýfinitioncf safe exposure lev-ls. At the present time the British Standards Institt.tion is revising its currentCode ;f Practice to -- ,vde =anufacturers of laser equipment with a set of standards and tne users of laserequloment with recommended operating procedures and non hazardous exposure levels.

Tne early Britirh Codes of Practice wnich were based on the limited experimental data tended to bes--_--.;.at -er-cautious in their approach to laser safety. Table 1 su=.arises the maximum permissibleexposure -evels at the cornea given in BS 4d03: 1972.

TALLE I

Energy dens:ty per pulse Power density

IASER TYPE Q-switched Long pulsed Continuous wave1 ns-lus pulse 1 .s-0. 1 s long-term

p.r.f. 10 Hz p.r.f. 10 Hz exposure

i Jcm..2 J/crr,2 W/cm 2

0x 6x 4xlO-7

(0.69 umj

*.eody m ium 2 x 10-7 3 x 10-6 2 x 10-6

(1.06 u-.)

Heliun-neon - 3 x 10-7(0.63 um)

Argon-0. 51 m)- 3 x 10-7

(').46 um)

As can :e s--n only three exoosare droations are considered, i ns - 1 us, a range which includes theQ-swltched e " es, I us - O. secund wnich covers the long pulsz systems and u.1 second upwards for

the continuous wave lasers. These values were applied to both the intra beam viewlng case where theretinal image diameter of between 10 - 20 microns was limited by the ocular aberrations, and to the viewingof extended sources and diffuse reflections. However a second table (not shown here) gave the correspond-ing safe retinal exposure levels and several instances occurred of safety officers attempting to redefinecorneal exposure levels for non-minimal retinal image sizes, without understanding the importance of imagediameter on damage thresholds.

'he more recent codes of practice ACGIH (Ref. 3) and ANSI (Ref. 4) have moved away from the conceptof providing safe exposure levels for specific laser systems and have peoduced an overall 'blanket' codewhich covers the complete range of wavelengths and all possible exposure durations. :hile there isconsiderable experimental threshold data available at certain specific laser wavelengths and exposuredurations where it is possible to make reasonably accurate estimates of the corresponding safe exposurelevels, the *blanket' type code by its very nature must still remain on the conservative side for themajority of possible laser parameters.

Current thinking on laser safety now places more emphasises on minimising the hazard rather than theaccurate determination of safe exposure levels. The hazard potential from laser systems can be groupedinto one of four main classes:-

Class I - lasers that are intrinsically safe: ie the maximum permitted exposure level cannot beexceeded under any conditions, or ! er systems that are totally enclosed and by virtue of their engineer-ing design cannot irradiate the eye or skin at levels in excess of the maximum permissible expoasr- levelz.

Class 11 - are low power visible lasers either continuous wave o. repetitive pulsed, which operate inthe visizle part of the spectrum between 400 and 700 nm that are not intrinsically safe but where eyeprotection is nor-mally afforded by the blink reflex.

Class ill - are those lasers which are safe only when viewed as an ex- ended source at minimum viewingdistance or wnose output when viewed via a diffuse reflector cannot exceed the maximum permitted exposzrelevel for diffuse viewing.

Class IV - are high powered laser devices which are capable of producing a hazardous reflection andmust be treated with extreme caution.

This system of classification which is carried out by the manufacturer or supplier of a laser systemallow the user to be able to identify the necessary hazard control procedures for s3fe operation.

Unfortunately most lasers of military importance will be classified as III or IV which means that theyrepresent a direct hazard to the eye (or skin) and where by nature of the mode of operation the hazard couldextend over several tens of kilometres. For in7tance a typical ruby laser range finder might have aswitched output of 100 millijoules with a eam divergence cf 1 milliradian, using the safe exposure ievelsquoted in the draft STA2JAG 3606 of 5 x 10 joules cm-2, the minimum safe viewing range can be calculatedto be 5 kilo=etres, and represents the nominal distance from the laser at which the naked eye would notreceive an exposure in excess of the recommended maximum exposure levýl. stis nominal hazard distance maybe modified by the use of optical viewing aids. Bincculars or weapon sites, unless fitted with absorbingfilters will increase the energy entering the eye Ly a factor approximately equal to the square of themacnification of the device. Thus, the use of xlO binoculars would extend the nominal distance of the abovesystez to 50 kilometres - an unacceptable range for :st practical purposes.

Tne "blanket' approach to laser safety while affording excellent protection to the majority of laserusers and members of the public may not always be relevant to the military where in some roles a iesscautious approach can be 3ustified. Two possible solutions can be found to this problem. The first, whichis outside the scope of this paper, is to define a hazard exposure level in terms of an acceptable overallprobability of producing a loss of visual function. The second is to base safe exposure levels on a moreaccurate estimation of threshold damage combined with realistic estimates of the eye's optical properties.The studies relating to the hazard from the Q-suitched neodymium laser affords an example of the secondmethod. Fig. 1 shows a relationsnip between th. estimated retit.al image diameter in microns and theretinal radiant exposure in joules cm-

2 for the ophthalmoscopic threshold (ED503. As can be seen, the

retinal radiant exposure increases with decreasing image size.

These variations of thres)hold Retinal Radiant Exposure (PPE) with image diameter had important impli-cations in the determination of tne safe exposure level for hum-ans. For practical considerations it isstecessary to express safe exposure levels in terms of the Corneal Radiant Exposure (CRE).

The regression, equation derived from Fig. I is

L0go RRE = 2.8149 - 1.0378 Olo0 dr(J.'cm2 ) microns

-1.0378or RRE = 0.5029 x d (d in millimetres)

r r

.CE = 0.5029 d 0.9622d As CRE = RRE dr pr

T-2p

In the major4ty of accidental exposures to laser radiation the incident energy can be considered asbeing perfectly collimated, resulting in a retinal image diameter that is limited only by the aberrationsof the eye and the state of accommodation at the time of exposure. At the present time as there isinsufficient accurate information on the modulation transfer function of the eye exposed to coherent mono-chromatic radiation and a simplified approach can be taken to establish a relationship between the various

9-3

optical parameters.

From geoma-tric optics it can be shown that the retinal image diameter arising from 4 eioptric errorAI ii of the form.

dL d (q- L) where q = distance from retina to refracting suriaceL and L = new focal length is given by

L =1000 (No-1) NL___________- (This term includes the dioptric error arising from a change in

(P0 +AP (MCI) wavelength,

P 0 is the dioptric power of the relaxed eye

N is the refractive index of transparent media atX 0 571.5

NL is the refractive index of transparent media at laser wavelength

A value for N1060 = 1.32578 has been calculated based on the information by Meyer et a.l (Ref. 5) basedon the "water model eye".

A va:. Meeteren (Ref. 6) has shown how the observed increases in longitudinal spherical aberrationsvary with pupil diameter. By replotting this information in a 3og for= a mathematical relationship can be

[. established:-

P = Kdns p

and the resulting retinal image diameter will be given by

ds dp q LI

L1

where L= 1000 (N - 1) NL(po 0+po0 +APs ) (NL - 1)

In this treatment both of the abovz aberrations arise from considerations of geometric optics and thelarger retinal diameter is taken as the limiting value. However diffraction is always present, and thetotal effective retinal diameter is taken to be:-

d = d where d is the greater ofd ane dr d + do 0 L s

and dd = 1.242NLq 0.022736

DpNL D

Values for CPS, based on the preceding assumptions have bte-n computed over a range of pupil diametersand states of accommodation, and are presented in Fig. 2. The minimum threshold corneal radiant exposureof 4 x l0-4 J cm-2 is based on the experimeiitally determined total intraocular energy required to producea 50% probability of causing an ophthalmoscopically visible lesion in the para =acular of the rhesus monkeyeye, and a factor of 20 should be included to exclude the possibility of damage at the electron microscopiclevel.

By comparison, it is of interest to note that if for a "near perfect" eye, if one assumes thatindependent of wavelength, a 7 = diameter pupil will produce a retinal image 10 microns in diameter, thenusing the da~a in Fig. I the corresponding safe corneal exposure level can be calculated as:-

60x 10 x I7 103 20

= 6. x10-6 j -2

This is in close agreement with the current safe exposure level (ANSI) of 5 x 10-6 J cm-2 for Q-switchedneodymium radiation.

Whichever safe exposure criteria is used, the nominal ocular hazard distance does not take into accountthe effects of the atmosphere on beam propagation-Middleton (Ref. 7) has shown how the effects of atmosphericatternation can be computed from a knowledge of the wavelength and the daylight visual range (Fig. 3). Forthe ruby laser discussed earlier, the effect of atmospheric attenuation for a visual range of 20 Km, wouldbe to reduce the hazard distance to 3.9 Ki, or if xlG binoculars were used, to some 17 Km.

Small changes in the refractive index of the air due to variation of temperatures near ground levelcan increase the eye damage probabilities by the break up of the propagating laser beam. To a viewer thisappears as scintillations (fluctuations in intensity). The hot upots or areas of localised beam intensifi-cation can have intensities that are 10 or 100 times the average beam intensity. At the present time thereis insufficient data to allow an exact calculation to be made oi. the probability of hot spotting, however,the effect is more likely to occur on ground/ground or ground/air operations at shallow angles of incidence(less than 10 degrees) and are more likely to occur around dawn or dusk when there are uniform changes in

Qg4

the atmospheric temperature. Additionally, hot spots are not likely to occur during overcast weatherconditions, in rain or fog or when there is a wind of more than 10 knots. When conditions favourable tothe production of hot spots, nominal hazard distances should be increased by a factor of 10, although thisfactor only applies to viewing by the unaided eye since large aperture optics tend to average out the hotspotting effects.

Within the next two or three years it is likely that international agreement will be reached on asystem of laser classification. The present Safe Exposure Levels based on ANSI Z 136, which are designedto protect laser users and members of the public, are unlikely to be modified in the near future. In theUnited Kingdom, the British Standard 4803 (under revision) will form the basis for all military laseroperations where non service personnel may be accidentally exposed.

REFERENCES

1. 'Protection of personnel against hazards from laser radiation" BS 4803: 1972. Issued by The BritishStandards Institution, 2 Park Street, London WIA 2BS. (1972).

2. Vassiliadis, A., Rosen, R.C. & Zweng, H.C. Research on ocular laser thresholds. Final Report ofProject 7191. Stanford Research Institute, California. (1969).

3. 'A guide for the control of laser hazards" Issued by the American Conference of CovernmentalIndustrial Hygienists: 1973.

4. "American National Standard for the safe use of lasers" ANSI Z 136. Issued by the American NationalStandards Institution, 1430 Broadway, New York. (1973).

5. Mayer, H.L., Frank, R.M. & Richey, F. Point dazage spread function for nuclear eyeburn calculations.Report No. 56321. Defense Atomic Support Agency, Washington. (1964).

6. A. van Meeteren. Calculations on the optical modulation transfer function of the human eye for whztelight. Report No. lZF 1973-2. Institute for Perception. Netherlands. (1973).

7. Beatrice, E.S. &Shawalak, P.D. Q-switched neodym.i" retinal damage in rhesus monkey. MemorandumReport M73-9-1. Joint AMRDC-AMC Laser Safety Team. Frankford Arsenal, Philadelphia. (1973).

8. Middleton, W.E.K. "Vision through the atmosphere'. Published by University of Toronto Press.

9-5

1000

S~500

CC

E200

z X

0 0

€02 ~ 50

Nx

20

of 02 0S 10 20 so 10 20 so 109

Rettin fOdiant exposure (J cm 2)

Fig. 1The effect of retinal image dia=eter on the ophthalmoscopic threshold retinal radiant exposure.

Data from Ref. 7. Ref. 2. Borland et al (in preparation)

SPupil diameter mm

30

1-6

E 14

4-0

- vi0

o 1-2

60

610 KdO"

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Fig. 2Computed values of 'threshold' corneal radiant exposure

versus dioptric error for Q-switched necdymium radiation.

6 3

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0nr .5a5 V isV=daylight visual range in YFm

2 eavelegtj In microns

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-Ip~•:auso h t•pe~ tenalnceE'ce• esswvlnt

OPHTHALMOLOGICAL EXAMINATION OF LASER WORKERS AND INVESTIGATION OF LASER ACCIDENTS

D.H. BrennanRoyal Air Force Institute of Aviation Medicine,

Farnborouqh, Hampshire, United Kingdom.

SUFW'ARY

Though ocular surveillance of laser workers is indicated from medico-legal considerations, theclinical aspects are equally important. These include assessment of personnel with pre-existing ocularpathology, detection of possible chronic effects and confirmation of the effectiveness of the safetyprocedures. Such surveillance is costly and it is important to restrict screening to workers involved withlasers capable of causing ocular damage.

Those aspects of ocular structure and function which are relevant to laser induced damage in man arediscussed. This includes the transmission and absorption characteristics of ocular tissues and the naturalprotective mechanisms of the eye. A scheme for the ocular surveillance of laser workers is presented withan evaluation of the role of field and other specialised examinations.

The procedure to be followed in the event of a laser accident is discussed. It is recommended thatthis involves a biophysical assessment of the accident scenario with particular reference to energy orpower densities which may have been incident on the subject's cornea, as well as a detailed ocular examin-ation of the worker. The clinical examination may include fluorescein angiography which has been found toLe a more sensitive technique for detection of damage than ophthalmoscopy in monkeys.

I• N-1RODUCiTI O

Lasers aie potentially hazardous to the eye and may lead to damage which could be confused with otherpathological changes. If such pathology is not discovered prior to a:cess to a laser hazard area diffi-culties may tise in a medico-legal context and so the primary reason for ocular surveillance of laserworkers tends to be of legal rather than medical importance. But there are other reasons, and these includethe early detection of an inadequate safety regime, advising personnel with disease in one or both eyes,such as a recurring uveitis, of the dangers inherent in a laser environment and the possible detection ofcumulative or long term effects of 'subthreshold' exposures to laser radiat. 3n. Though it may appearunsatisfactory that surveillance of laser personnel rests on legal rather than medical grounds, experiencein t:his field e=phasises that an environmental vision programme is expected not only by laser workers butalso by industrial medical officers.

Tne main difficulties in establishing an ocular surveillance programme are cost and the small -umberof ophthalmologists with experience of laser induced pathology. This situation may be remedied by coursesof instruction on lasers and their hazards, it is important to restrict surveillance to workers who areinvolved with lasers which can cause damage as defined in current codes of practice such as BritishStandards Institute 4803 (at present under revision) and American National Standards Institute Z 136.STANAG 3606 (Ist Draft Edition no. 2) classifies laser systems hazards as follows:

a. Class I - Exempt. If the total output power or pulse energy concentrated into the limitingaperture, ie. 7 m for 1.4 um or I mm for 1.4 um, which could occur during intrabeam viewingwith a magnifying optical instrument, does not exceed the appropziate Protection Standard at thelaser transmitter optics exit aperture, then the laser system is classified: Class I - Exempt.Tnis implies a nominal ocular hazard distance of zero and thus no further hazard evaluation isneeded on Class I systems. See, however, Class IIIa below.

b. Class II - Low Power. The following laser systems are classified: Class i" - Low Pcwer:-

(i) Visible (400 nm to 700 nm) CW laser devices with output power greater than 0.4 W but equalto or less than I mW.

(2) Visible repetitively pulsed laser devices which can emit a power exceeding the product of

the appropriate intrabeam Protection Standard multiplied by the circular area of the limitingaperture (see para. a. above) for tbe maximum possible duration inherent in the system, but notexceeding that figure for a 0.25 s exposure.

No further hazaid evaluation of Class II laser systems is required.

c. Class III - Medium Power. These are laser devices which emit radiation that is hazardous to viewdirectly or after specular reflection, but not hazardous after reflection from a diffuse surface.The fc'.owing laser systems should be classified: Class III - Mediva Power:-

(1) Single-pulsed lasers of wavelengths between 400 m and 1400 nm if the radiant exposure (H)per pulse at the transmitter optics exit aperture (= output energy (Q) per pulse/beam ares) exceedsappropriate Protection Standard, but falls below the values for diffuse reflections.

(2) All CW lasers with power outputs greater than 1 mW but not exceeding 0.5W.

(3) For repetitively pulsed lasers of wavelengths between 400 rm and 14C0 nm and of PRF greaterthan I Hz, it is necessary to determine both the CW and single pulse Protection Standards and thento apply the more stringent in evaluating the hazard. If the beam irradiance (E) or radiant

n -r-. . . . .. . .. . . . .. . .. . n - -I .. . . . .. ....... .... .. .. ...- - - - - - - - - ---|i i ... U n a

'2 10-2

exposute (H) at the transmitter optics exit aperture exceeds the selected Protection Standard but* falls below the limiting value for extended source viewing, then the laser system falls in this

3 Cla.is. A special Class Ilila is applied to laser devices which for intrabeam viewing with theuPniideJ1 eye appear to conform to the criteria of Class I - Exempt, but where the ProtectionStandard is exceeded for viewing with magnifying optical instruments.

*d. Class IV - Hiegh Power. These are laser systems which emit radiation that is hazardous even after* reflection from a diffuse surface.

with this classifiration surveillance should b4L restricted to workers operating Class III and IV lasors.Personniel operating potentially hazardous lasers may be subdivided into those at high and those at low riskaýccording to the nature of their duties. This division would not influence thz. type of examination butwould modify the frequency of examination. But it may well be that those considered of low risk will bemore careless or take more cninces than those considered of high risk.

Tissues at Rifk.* In order to appreciate the manner in which laser radiation may affect the eyes it is ofvalue to have an understanding of basic ocular structure and function.

* The eyeball is roughly spherical and approximately 2.5 cm- in diammeter, It lies within the bony orbitsuspended in fat. it is protected fro= damage in all. directions except anteriorally where protection islimzited to that provided by the lids. The eye rotates about its own centre in response to the pull of theextraocul.±r muscles.

ine goboke consists of three coats which are =modified at the front to admit lignt. The outermost coator sclera is tough and supporti-ve, the anterior transparent region is called the cornea. The middle coator uvea is vascaolar and its prime function is nutritive. Anteriorallv this coat b~ecomes the ciliary bodyand iris wnereas posteriorally it is %nown as t-ne choroid. 'The innermost coat is neuroepithelial and calledthe retina, it is lignt sensitive and i.-. extent corresponds to the choroid. The hollow globe is dividedinto Iwo compartzlents, a small anterior cna- er filled with a watery fluid called the aqueous, which islimittd my the cornea and lens iris diapnraom and a larger posterior compartm-ent which is filled with aclear jelly called thle v;treous and is bounded my the len.s iris diaphragm and the retina (F'ics. I & 11).

4 T~ie anterior transparent window or cornea is approximately 1 =m thick at its junction with the Sclera- tninning to approximately .7 cm at its cent~re. It is com-posed of 4 layers, a thin outer epithelium which

-.ill reg;enerate if damaged. a thick layer of fibrous lazlellae which is called the substantia propria anda n~n inner elastic memorane called Descemnet's membrane which is coverec- wit.ý an endothelial coat of single

cell thic~ness and is continuous with the endotheliu= of the anterior surface of the iris, if damage isc')nfined to tne epitheliiz full recovery should takLe place within 48 hours but if the deeper structures are:nvolved an opaque scai may result. The cornea in comon with skin and con3unctiva is at risk primarily

v ~to -asers operating in the far-infra red atmove 1.4 ..icrons. or In the near ultra v: ,let below 400 nm. At

teewa.e~engths all otological tissues are oraque (i.I). hecornea is transparent to wavelengthsLelow 1.; microns and ab~ove 400 nm and pro-vides the majoritv of the refractive power of the eye having anapproxim-ate power of -43:.

Tne iris is a pigmented contractile tissue which has a hole in its centre called the pupil. Thecolour of the iris is a function of tnqe degree of pi-'mentation. o-rown epos being heavily. pigm-ented, whilst:m._e eyes are only lightly pigmented. -Tne pazillary diame-ter vai~vs in Size With the contraction of theiris =uc.lature. tnt. norm-al excursion neing Ltween 3and 7 =m but with drugs this can increase to 1.5 -

The pupil varies in size according to a.mn)en lighting and iegulates the amount of light enteringtne eye. this oein, prrport~onlal to the square of tie pupillary diameter. Thnis regulation will only applyto Lea= sizes greater than the pup.-ilar-y d-amecter. Thne iris will absorb energy incid,-nt izpon it. the degree*.f c:-sorptzon being related to tne degree of pigm-entation. Lasers in tne visible and near infra-red regionsmay Cta.5e iris damage (Fig. Ill).

.ne - lcens is the second refracting surface- providing approximately *~,~when c'PaccO-,ocm aLed withCa erof a cmorXd~atxon wnicht decrease,. witrn agc bioing approximately -ND at 10 ye~ars, -4D' at 40 years

and ':,at ý,O years. T:.e lens continues tile refraction started by thne cornea and brings rays of lightZo a fo'C's 0%: til. retlna. The lens usuplly suffers danage :)y heat beting con'1icted from the iris with whichit lies - a~t<.siticn although energy from some lasers mady De absorbed directly in the lens substance.

Ais the ref ract:ve components -,f tine eye form a rough.y hociocentric systeiz. it is possible to regardone'eye's fo'cusing -techanism as being a single refracting surface. The reduced eye as proposed by Listing

4 r.~inr -,,tns surfatc- as being :.5 = behind the cornea and having a radius of curvature of 5.7 =m. it liesmetween- 2 media zrossessing refractive indi-:es of 1 and 1.336 and its anterior and posterior focal lengthsare :7.ý =m and 22.9 r--. WiItn these ficures it is possibie to calculate the nature of images formed on

hL .n rettina is a thin. transparent membrant- which covers the posterior com-par-rA-ent except anteriorallyr ~ ana wt.cr-. it is pe~rforated my the optic nerve. The outermost layer of the retina is the pigment epithelium

and tinis lies o:. tne choroid, inner to the pigment epitheixum- atf- the lineht sensitive receptors called rodsan d rznes. Inrto the rods and cones ..re the Dipolar cells and their Synaptic layers and innermost -,f all.are t.e, ganglion cells and theii axons whl-cin converge on thv optic diIs: IFie-. 1.). There is a special iseacra of ret'ria k:.ow.. as tne m:acula with% tine foveal pit at its centre. This area lies approximaitely 3 m

eIprito tine .pit:c disc and is used for all tasks demanding nigh visuai acuity both form and colour,a-nc 1s a:. area com~posed entirejy of cones. Th~e rods win~ch increase in density peripheral -c the fovea.zrL- usc-z fsr n:-ght vj:si',r. :,ing highly lignt sensitive winen adapted hut tney cannot differentiate. colours:,r provi'1e: ; qoA, form acuity. if one constders fo~lacuity as unity it, is 'ound that it~ 50 eccentricfr=tm ue fovea tinr acuity has dropped to 0.25, at 200 eccentrxc to the fovea this figure has fallen to O.0S.:t is -a=,,gc to tine -xacular region with whichl we are most concerned as it. is cell' damage here which callsest:.e ;rltst pre.found effect, on vision.

10-3

As can be seen from the retinal structure (Fig. II) light must pass through the nerve layers cf theretina before reaching the light sensitive receptors; these rods and cones lie in intimate relationship

r with the pigment epithelium which envelops a portion of their outer segments. Energy from the visibleand near infra-red lasers traverses the retina and is absorbed by the melanin in the pigment epitheliumand choroid. This rapidly heats and by virtue of its intimate relationship with the receptors causes theirdamage. The extent of this damage will be dependent upon the energy reaching the pigment epithelium, thearea irradiated and also on the degree of pigmentation present, which may vary racially.

As can be seen (Fig. III) there is a secondary absorbing site at the macula in the inner plexiformand nuclear layers where the blue/green wavelengtl.s are selectively absorbed in the macula pigments. Theseblue/green wavelengths produced by the argon and dye lasers are also absorbed by the haemoglobin of redcells within the retinal vasculature.

To stumarise, the eye is adapted to refract light in the wavelength band 400 nm - 1400 nm and theprimary hazard from lasers operating in this region is to the pigmented structures within the eye. Thepigment epithelium is the tissue most at risk and heat generated here may be conducted to tie receptorsgiving rise to a scotoma or blind spot. The other pigmented tissues at risk are the choroid and iris.Heat may also be conducted from the iris to the lens, with which it lies in apposition, producing a"cataract. The macular pigments and haemoglobin are secondary absorbing sites for the blue/green outputof argon and dye lasers. Tne przrary hazard from lasers operating outside the visible and near infra-redwavelengths is to cornea, skin and con3unctiva.

"natural Protective Mechanisms. The eye possesses protective mechanisms which may assist in limi~ing laserdamage. Lacri=al fluid in common with biological tissue is opaque to the far infra-red wavelengths and toa limited extent the tear film will absorb and dissipate energy incident upon it. The cornea is richlyinnervated and any damage causes intense pain and triggers the sensory blink reflex within approximately.1 second, thereby limiting further damage. Bright light from visible lasers will stimulate the opticalblink reflex but this is even slower than the corneal reflex and does not provide protection againstpulsed lasers but it may be of value with continuous wave lasers. A bright working environment may helpto protect the retina by ensuring that the amount of energy entering the eye is limited by a small pupillarydiameter. Normal eye movements, tremors and microsaceades, whilst again too slow to mitigate damage frompulsed lasers may be of assistance with continuous wave and repetitive lasers by spreading the energy overa wider area. The optical quality of the human eye is such that spot sizes smaller than 10-20 microns are

nýikely to be achieved and it is generally assumed that the maximum optical gain from cornea to retina is4.5 x 105, approximately : million timer.

Differential Diagnosis. The appearances of a laser burn may closely mimic a variety of normally occurringocular pathologies. The list of diseases which may offer confusion with laser induced eye damage is legionand includes any condition which can cause areas of blanching, oedema or pigment clumping. A few exampleswill be cited. A rezinal burn can resemble a Local choroiditis, a central serous retinopathy, an eclipseburn or a macular dystrophy. Lens damage can result in cataracts which may .losely simulate those arisingcongenitally, from trauma or in senility. eurns of the iris .an resemble an melanoma whilst a corneal Lurnin its later stages may ;,uudCc= . zebulý which mey be indistinguishable from those arising from ulcerationor dystrophy.

Examination Protocol. It is important to ensure that the examination protocol for workers at risk, fromhazardous lasers is both relevant and realistic. Given the diversity of wavelengths at which lasers canemit, all ocular tissues are potentially at risk.

M.e output of lasers which operate in the near ultra-violet and the fat infra-red is absorbed by skin,

conjunctiva and cornea. if a worker is solely involved with lasers emitting in these regions it is onlynecessary to examine the ocular adnexa and external surfaces of the globe with a loupe, particular attentionbeing paid to a corneal examination using a slit lamp. The slit lamp comprises a low power microscope witha light source which is capable of producing an optical knife section. It is possible to focus at differentdepths and thereby examine in detail the transparent media and irLs. The slit lamp techniques of retroreflection and specular reflectior. may al-o aid in d-,monstrating minimal damage which might otherwise remainundiscovered.

The examination scheme suggested for workers who are involved with lasers which may lead to intraocularas well as damage to the external surfaces, is necessarily nore detailed. However all examinations shouldbe reduced to the minimum and all hazardous or unpleasant procedures deleted where possible.

It is unlikely that a laser burn would increase intraocular pressure and so tonomntry need not beincluded unless indicated. Similarly scleral indentation and examinations with a mirror contact lens andother examinations to visualise the retinal periphery are disliked and of doubtful value. Field examin-ations are time consuming and as scotomata produced by lasers are likely to be large and obvious or small,of around 10-30 microns, and difficult to detect, campimetry and perimetry have not been ,ncluded as aroutine. It has also been suggested that tests of ocular muscle balance should be undertaken but again itis most unlikely that lasers could cause an-, alteratic.i in tropias or phorias, and the value of such testsis doubtful.

The examination proposed at Annex A attempts to assess the worker hazard both in terms of lasers usedand his particular duties. There follows an enquiry into his ocular and general medical history, particularattention being paid to entoptic phenomena such as the development of after images, blind spots and alter-ations in vision both form and colour. The objective part of the examination is concerned with the externalappearance of the eye and adnexa together with tests of pupillary function. This is followed by mydriasiswhich although inconvenient is considered necessary and a slit lLmp examination of cornea, iris and lensand lastly an ophthalmoscopic examination of the fundus particular attention being devoted to the appearancesof the posterior pole. Any pathology is documented, preferably photographically and in the normal eye 3fundus photograph of the posterior pole including the optic disc and macula is considered desirable. Anyfurther objective tests are left to the discretion of the examiner being based on his findings and opinions.

10-4

The subjective examination comprises te. ts of central and paracentral function, as it is burns of the=acula affecting central functior which would cause a significant disability. These include tests ofvisual acuity for near and far with a refraction where necessary. Colour vision is tested using thepbc-ido isor.hromatic plates or an approved lantern subtending a visual angle of I - 30, as it ia possiblethat cclocred lasers might selectively damage one type of cclour receptor when below burn threshold.Paracentral function is tested by means ol the Amsler charts. The Amsler grid in its simplest formconsists of a black card printed with a white grid pattern, this is held 30 cms from the subject's eye.The subject fixates a spot in the centre of the grid and at 30 cms the whole grid subtends a visual angleof ipproximately 100 around the fixation point. Each eye is tested in turn and the subject is asked sixstandard questions.

Question 1 - Do you see the white spot in the centre of the squared chart?

This question detects the presence of an absolut. or relative central scotnma. if the subject onlysaw the fixation point when he looked off centre, it would reveal the presence of a foveal burn. Thiswould be a severe disability.

Question 2 - Keeping the gaze fixed 'pon the white spot in the centre, can you see the four cornersof the b-; square? Can you also see the four sides of the square? In other words, can you see the wholeof the square?

This question does not have a great relevance in laser screening but could detect a scotoma coming infrom tne side such as the arcu:ate scoto-ma of chronic glaucoma, which might offer confusion.

Question 3 - While keeping the gaze fixed always on the central fixation point, do you see, in thewhole square, the network intact? Or are there interruptions in the network of squares, like holes orspots? Is it blurred .n any place? And if so, where?

Th~s que.;tion reveals the presence of a paracentral scotoma absolute or relative anywhere, except thefovea, witnin the area of retina tested. It is the question of greatest value in laser screening.

Questi ons 4 and 5 - Always keeping the gaze fixed on the white spot in the centre do you see all thelines, both horizontal and vertical, quite straight and parallel? In other words, is every small squareequal :n size and perfectly regular?

Always fixing the gaze upon the centre point, independently of blurred spots and distortions, can yousee anything else? A movement of certain lines? A vibratior or wavering? Anything shining? A colour ortint? And if so, where on the square?

These questions reveal the pres, nce of metam.orphopsia and entoptic phenomena such as might beproduced by s=all degrees of retinal oedema fiom heat or selective cone destruction by a coloured lasercausing da=age restritted to the photcchemical leve

Q*.estion 6 - Keep:ig the central point fixed, at what distance from this point do you place the bluror distortion you see? How =any small intact squares do you find between the blur or distortion and thecentral point hat you are keeping yojr gaze upon?

Tnis question a,*curateiy locates camage in relation to the fovea.

Great :=pcrtance n,-, been attacned to the Amsler test as it is considered to be of great diagnosticvalue a.n rap:O in use.

Fluorescein. Angiography. Fiuorescein angiograpny has proved to be a reliable and sensitive technique fortne detection of laser damage to the retina. In animal studies using the rhesus monkey it has proveo tobe an>out 6 ti=es more sensitive tnan -phthal.zoscopy in the determination of the 50% probability or damagefor tne Q-switched neody-ium laser 4;1-:. ., 'is:r.an, D.H. (Fig. IV).

in man 3 cc of sodiur. fluorescei:, in a 20-25% solution are given by rapid intravenous injection andserial photography is commenced as soon as fluorescein illuminates the fundus and continued at tppropriateintervals for up to ten =inutes thereafter. The equipment in use at Farnborough comprisos a Zeiss (West)fundus c:aer4 with a Baird Atomic B5 exciting filter end an Ilfori iO0 Delta chromatic 3 barrier filter inthe mctorlsed magazine. These filters allow only about 1% transmrttance in the overlap zone of 480-500 nm.The f11= used is Ilford FP4 which is developed in Kodak D76.

The background fluorescence varies with phases of the vascular cycle. The first fluorescence seen istne cnoroidal flush when the dye first reaches the choroid. This fluorescence is patchy and irregular indistribution, it is followed by the arterial phase when the fluorescence assumes a fine granular patterndue to the dye in the choriocapillaris being viewed through discontinuities in the pigment epithelium.Fluorescence becomes maxzmal during the early venous phase and then commences to fade away assuming oncemore a granular pattern which becomes coarser with the passage of time. It is during the later venousphase that fluorescent laser lesions are most readily seen.

The ophthalmoscopic appearance of fluorescent lesions depends on whether they have been produced bya near threshold or sipra threshold exposure. Threshold lesions fluoresce uniformly during the venousphase but lesions above threshold appear as a ring pattern during the early venovs phase and fill slowlyfrom the periphery toward centre during the late venous phase. Large fluorescent areas in excess of75 microns are easily seen when superimposed on the background granularity but small lesions less than75 microns are more difficult to see as they can be more easily confused with background grain.

In lesions at threshold levels the junction between adjacent pigment epithelial cells which are calledzonular occludens become separated due to thermal damage and this o~ening represents a break in the chorio-

10-5

retinal barrier and permits free diffasion throughout the irradiated area and the lesion fluorescesuniformly. In lesions at 4bove threshold level the pigment epithelium becomes coagulated and thusimpermeable to fluorescein except at its periphery where the coaqulated shrunken central plaque pullsopen the ]unctions between normal and coagulated cells giving rise initially to the typical ring pattern.The ring slowly infills from the periphery to the centfe with the passage of time.

Accident Procedure. In the event of a suspected laser accident the worker should be examined using thesame protocol as detailed in Annex A. This examination should be conducted as soon •s pssible after theevent preferably by the same ophthalmologist who carried out the original screening. In equivocal caseswhere damage cannot be excluded or where the extent of damage is difficult to assess fluorescein anglo-graphy is of great value provided this is done within 4C hours of the event.

When an accident is suspected the site of the incident should be 'frozen' until after a biophysicalexamination. This would attempt to determine whether the power or energy densities which had been presentat the workers eye could have caused damage. This information could be of great value not only medico-legally but also in relating damage to energy levels and assisting in the development of new codes ofpractice.

REF ERENCES

i. BReNAN:, D.H. Ocular examination of laser wor~ers and investigation of accidents. Royal Society ofMedicine, 66: p9-9, Sept. 1973.

!L

2 10-6

Conjunctiva

Aquc'ius Humour

Z2 Pupil (black area)

Sclera (white of eye) Ln

Retinal Ves Fve

4 -e ia

Arter

10-7

Blood Vessel

Conjnct i T Nerve Fibre LayerAqueousGanglion Cell LHumourInner Plexiform L

ft Inner Nuclear LEED~Outer Plexiform L

SReceptor CellsIris (rods & cones)

EpitheliumCh oriocapi liarisChoroid

Inner PlexiformOptic FbeLyrLayerDisc of Henle Inner Nuclear L

Receptor CellsFovea(cones)

Epithetlium

Fig. Uii

10-8

Blue - Green

Far Inf ra - Red LUltra -Violet 0 C

NonIn -a pigmnte

Blue - Green

Fig. ME

7vP77 - 77. ~

-~ 10-9

Fig. IV

..ýor :scexir angiogram of rhesuis nunk-y ret.,naw-Itn fluoreScent laser lesson~s between opt~c o~ss and macula

10-10

OPHTHALMIC SUPERVISION OF LASER WORKERS

Examination date ...................... Date of star'ting/ending laser work ..........

Name .................................................................. Age .........

Address .............................................................................

Place of work .......................................................................

Laser Type * Maximum Output Class Special Features

Worker Hazard Rating High Medium Low

Delete above where applicable

Ocular history ...... ...............................................................

,.................................. .... .. .........

Entoptic ..en .ena ................................................................

S...... • .° .... °°..°. °.. ° ...... ° .... .. °°.... ...........................................

Relevant general medical history ....................................................

Tick where applicable

Right Left

External Appearance: Normal I Abnormal Normtl Abnormal

1. Lids .................................2. Conjunctiva .................3. Cornea4. Sclera . . . . . . . . . . . . . . . .5. Iris . . . . . . . . . . . . . . . .6. Pupillary size7. Pupillary react:.ons .......

Near Far Near

9. Visual acuity with correction . . .Correction prescription

Refraction if V.A. achievable less than6/6 6/6 =6/

jNormal Abnormal Normal Abnormal

10. Ansler grid ..................

Colour Vision:

11. Lantern (1-3 minutes visual angle) .................................

and/or

12. Pseudo ochromatic plates

Accepced Refused

13. Mydriasis

_ • l I t ,:-. , . . . . i - •-,-• - ii i • i i l

Right Left

Normal Abnormal Normal Abnormal:÷ ~Slit Lamp:.,

14. Cornea .......... ................15. Iris16. Lens

17. Fundus

Taken Not Taken Taken Not Taken

18. Fundus photograph of posterior pole

Nigh Med Low High Med Low

19. Ocular pigmentaton

Additional examinations at discretion of

exalminer. e.g.2o. Central fields

21. Applaration tonometry22.

,23.

24.

zjarrative description of any abncrma)ities discovered, accompanied by photographs ordrawings where applicable.

Examiner's Name ......................

Signature ................ .....

Wurk"is who ar' restricted to the use of lasers operating solely in the infra redwavelengths, above 2 um e.g. carbon dioxide lasers, may have their examinationslimitet to the ocular adnexa and cornea.

4;Il .. ... . . . .

LASER PROTECZIVE DEVICES

DAVID H. SLINFY

LASER-MICRCWAVE DIVISIONUS ARMY ENVIRONMENTAL HYGITENE AGE':CY

ABERDEEN PROVING GROUND, MARYLAND 21010

SUM.MARY

Since the eye is the most vulnerable site of injury for visible and near-infrared laser radiation, theprimary interest in protective devices has centered on eye protection. The ideal characteristics of lasereye protection will be presented and the present filter materials and goggle designs will be comnared withthe ideal. Ultraviolet and far-infrared radiation can cause injury to the skin as well as to the eye atcomparalae exposure levels; hence the skin requires protection from lasers emittino in this region,although protection of the eye remains paramount.

1. 1, RODUCTION.

a. Most industrially oriented laser safety codes emphasize the most desirable laser hazard controlmeasure: the complete enclosure of the laser system. However, this is not always practical and is"effectively impossible for military applications. Laser eye protection generally offers the bestalternative to beam enclosure for military laser use in the field. For some laser maintenance prcceduresand for constantly changing experimental arrangements in the research laboratory, eye protection providesthe simplest solution to the laser safety problem. Protection of the skin is seldom necessary and I willconcentrate therefore, on eye protection.

b. Several factors play a role in determining whether eyewear is necessary and, if so, selecting theproper eyewear for a specific situation. At least three output parameters of the laser must be known(maximum exposure duration, wavelength, and output power/energy; or maxintun exposure duration, wavelength,and output irradiance/radiant exncsure). Additionally, knowledge of ervironnental factors such as ambientlighting and the nature of the laser operation is also required.

c. Laser eye protection generally consists of one filter plate, a stack of filter plates, or twofilter lenses which seleftively attenuate at specific laser wavelengths but transmit as much visibleradiation as possible! . Ey-c.ear is available in several designs--spectacles, cover-all types withopaque side-shields, and coverdlly types with somewhat transoarent side-shields (Figure I).

1i

Figurt 1. laser Eye :'rotection Cames in Many Varierics.

2. APPLICATIONSS.

a. In the indoor shop or laboratory environment, eye protection is required for unenclosed"high-pokwer lasers" which are pulsed 1asers which present a diffuse-reflection viewino hazard, or CWlasers having a total power above 0.5 w.

b. Several laser applications exist in which a potentially hazardous laser beam is propagated in theoutdoor environment. Some construction applications, atmospheric research, and air pollution monitoring,as wdell s military applications fall into this category. In these applications, steps are taken at firstto prevent individuals from entering the beam path or the laser frock enterina occupied areas, and eyeprotection is used as a last rebort. Eye protection is extensively utilized where individuals must be"downrange" within the beam path as in some atmospheric laser bean propagation studies, lasercounication experiments, and in two-sided tests of military laser rangefinders and designators. 2 If onewere directing a lawr at a specular target during a test or training exerc:se, eye protection for allwithin the hazardoL: envelope would be required (see Figure 2).

DIRECT BEAM

HAZARD. /

ENVELOPE.~N

/ /

//

[ 'N

LASER •

Figur-. 2. The Potentially .azardo-s Sp-ecular Reflection Zones Depend Upon Laser Beam Polarization. Ifthe Electric Vector of the Incident Light is Parellel(1) to the Plane of Incidence. then theZones are Restricted.

/ )

Il ~ mm m .m L•

11-3

c. Before deciding that laser protective eyewear offers the best solution for controlling potentialhazards, one should consider alternative controls while being aware of the disadvantages of such eyewear.Most laser protective goggles are somewhat uncomfortable to wear for extended periods of time; lenssurfaces fog in most environments. many goggles provide only "tunnel" vision, or at best, reducedperipheral vision; they reduce visibility; and they reduce total colour perception, as well as possiblyrendering warning flares of certain colours non-visible. Reduced vision from eyewear can introduceincreased risks in many occupations, for example to aircraft pilots. Moreover, an individual wearingeyewear in the vicinity of a laser beam path introduces an additional risk from specular reflections toany unprotected bystanders.

d. We have considered the risks of hazardous specular reflections from our own lasers in a combatenvironment. In our judgment, they are so small that we do not plan to provide eye protection for combattroops against this hazard, since the shortecomings of goggles outweigh this risk. On the other suand,with lasers directed at combat troops, there may be a sufficient risk to warrant goggles for troops in ornear hard-point targets. Certainly in any test or training environment we require such eye protection.Wnsidering nominal hazardous ranges and levels of ocular exposure at typical engagement distances leadsus to this conclusion; the individuals at great risk are those viewing the laser source with opticalinstruments from within the beam. We now have protective filte-s built into the optical sights of somecombat vehicles. These filters cna employ dichric coatings with much hiqher visible transmittances thancould be used in individual safety spectacles.

3. LASER VIEWING EMNCEMENT GOGGLES. Several commercial manufacturers have offered goggles designed toselectively transmit, rather than attenuate, at a specific laser wavelength. These goggles were designedfor use with helium-neon lasers used in daylight in the constuction industry, to permit workers to readilylocate the beam at much lower irradiances than would otherwise be possible. This type of goggle has notas yet found any use in the military environment. Obviously, if such goggles are on hand, the eyewearmust be clearly marked that they do not offer eye protection.

4. "ARX.'XTERS OF LASER EYE PROTECTION. Several physical parameters are useful in provieing an adeq..atedescriptl.n for specific eyewear:

•a) Wavelength. The wavelength (s) of laser radiation limits the type of eyeshields to those whichprevent the particular wavelength(s) from reaching the eye. It is emphasized tl~at many lasers emit norethan one wavelength and that each wavelength must be considered. Considering the wavelength correspondingto the greatest co.tput intensity is not always adequate. For instance, a helium-neon laser may emit 100mw at 632.8 = and only 10 6W at 1150 nm n=, Dut safety goggles which absorb the 632.8 mu wavelength mayabsorb little or nothing at the 1150 wavelength. The only comonly encountered 1-ised lasers whichintroduce this problem are frequency-doubled laser systems, suc.i as Nd:YAG, which will have both 1064 mand 532 =u emissions.

(b) Optic-' Density. Optical denisty is a parameter for specifying the attenuation afforded by agiven thickness of any attenuating filter. Since laser beam irradiance may be a factor of a thousand or amillion above safe exposure levels, Percent tran&mission notation can be unwieldy. For instance, filterswith a transmission of 0.000001 percent can be described as having an optical density of 8.0. Opticaldensity D), is a logarithmic notatio.o and is described by the following expression:

D. = logio Eo = -loglo tr.E

where Eo is the irradiance of the incident beam and E is the irradiance of the transmitted beam ofwavelength ;.. Thus a filter attenuatinr. a beam by a factor of 1,000 or 133 has ar optical density of 3,and another filter attenuating a beam by 1,000,000 or 106 has an )ptical density of 6. The requlredoptical density is determined by the r-aximum laser beam irradiance to which the individual could beexposed. The opt.cal density o:. two highly absorbing filters when sta,.ked is essentially- the su of twoindividual optical densitities, but not exactly.

(c) The total transmittance of an absorbing optical filter is ths product of the internaltransmittance of the absorbing medium (which is dependent upon the filter thickness) and the transmissionlosses due -o Fresnel reflection at the filter surfaces. Hence, two stacked filters bonded with opticalcement will have slightly less density (.0.04! than if separ.ted.

(d) The upectral transmittance of glass or plar-ic filter materials is generally obtained from athin-filter sample which has been molded or ground zo a useful thickness to provide no less than I percenttransmittance within the wavelength band of interest usinq high quality spectrometer. The opticaldensity for the raterial of a givon thickness ±2 at a given wavele-ngt) may then be calculated from thettansmittance z, of the sample c kness ,I if the Fresnel reflection component is adequately accountedfo.- 10. For a beam incident perpýi ar to the filter surfi.-e, total transmittance T is the product ofthe internal transmittance t1 (depereent tpon thickness) and the reflection loss R which itself isdependent only upon index er refraction.

S= R-:.= 2n

T"lie Density D. " -'og Ti log 10 R -logio T (3)

and

Di tY =

D0 (t 2 ) t2

4 11-4

For example, if a 2--a thick Schott9 BG-18 filter"I has a density due to internal attenuation of 1.96(2.00 total density at 694.3 nm), then a 5-mm thick OG-18 filter would have a density of 9.8 due tointernal attenuation, plus 0.04 due to reflection, hence 9.84 at 694.3 nm.

4.3 LASER BEAM IRRADIANCE OR RADIANT EXPOSURE. The maximumi laser beam radiant exposure in Joules-CM- 2 forpulsed lasers or maximum laser beam irradiance in Watts*cm 2 for continuous-wave lasers to which anindividual may be exposed cannot always be readily determined. If the beam is never focused and is largerthan the diameter of the eye's pupil, the output energy-per-uni*.-area (radiant exposure) ofpower-per-.mit-area (irradiance) should be the guiding value. If the beam is focused or if the beam canbe observed directly through binoculars, the maximum total bean energy or power output must be used.

5. VISUAL TRANSMITTANCE OF EYEWEAa. Since the object of laser protective eyewear is to filter out thelaser wavelengths while transmitting as Puch of the visible light as possible, visible (yr luminous)transmittance should be noted. A low visible transmittance creates problems of eye fatigue and mayrequire an in.crease in ambient lighting in maintenance-shop or laboratory environments. However, adequateoptical density at the laser wavelengths should not normally be sacrificed for :Uproved visibletransmittance. ror nighttime viewing conditions, t.%e effective visible transmittance will be differentsince the spectral response of the eye is different. Figure 3 shows the CIE 'standard observer's scotopic(night vision) and photopic (day vision) responses of the eye according to the Ccmmissicn Internationale

do L'Eclairage 2 . 7hese are mathematical functions that attempt to show the approximate spectralsensitivities of the eye for two types of human vision. They are probably the extremes of acvual viewingconditions encountered with laser eye protection. Certain colored filters would therefore, affe-tdaylight vision differently than night vision. Blue-green filter lenses such as BG-18, which are used toprotect aga-nst •ruby and neodvvmium lasers, therefore, have higher scotopic transmission values than red ororange lenses, and vice-versa.

1.0

U SCOTOPIC"

U_

i0z .

.4

WAVELENGTH (n n)

figure 3. The CIE Visual Sensitivity Fu.rtion of the Eye for Daylight Conditions (Photopic) and;i-attime Conditions is shown at left. The relative spectral transmittances of severalprotective filters of military interest are shown for comparison in the right-hand panel.

6. lASER FILTER ')AjkGE THRESHOLD (HAXIMUM IRRADIANCE). At very high beans irradiances filter materialswhich absorb the laser radiation are damaged, thus it becomes necessary to consider a damage threshold forthe filter. Ty.pical damage thresholds from q-switched and mode-locked pulsed laser radiation fall between10 and 100 Jcdles-om- 2 for absorbing glass, and I to 100 Joules-cm2 for plastics and dielectric coatings.Irradiances from CV lasers which would cause filter damage are in excess of those which would present aserious fire hazard, and therefore, need not be consideted, i.e. personnel should not be permitted in thearQa of such lasers. Figure 4 shows examples of damage to laser filters from intense laser beams.Generally, only surface effects are noted, and little change in optical density results. Plasticmaterials melt superficially, glass surfaces craze, and dielectric coatings vaporize.

Figure 4. Da-iage to a dichroic ccating (a) on a filter is normally in the form of pin holes created atl-1OJ-cs 2 . Daxrage to glass; (b; is in the fo~rm of surface fractures at l0-100OO~cli 2 .

- Damage to plastic is typically surface

11-6

7. FILTER CURVATURE. If curved protective filters are required for personnel in a laser target area,personnel in the vicinity of the lamer and elsewhere would not also require eye protection. Potentiallyhazardous specular reflections can exist to significant distances due to the preservation of the bea'scollimation from flat-lens surfaces as can be seen in Figure 5. Hence, the curved filters are far moredesirable than flat lens filters. 7he use of the standard six-diopter curvature on spectacle lenses alsoreduces visual distortion.

1 CURVED

FILTER

LENS

FIGUSRE 5. The Specular Reflection of a Collimated Beam from a Flat Surface Retains its Collimation butfrom a Curved Surface Diverges.

8. METHODS OF CONSTRUCTION

a. There are basically two effects which are utilized to selectively filter out laser wavelengths.Filter are designed to make use of selective spectral absorption by coloured glass or plastic, or.elective reflection from dielectric coatings on glass, or both. Each method has its advantages.

b. The simplest method of fsbrication is to use coloured glass absorbing filters which are qenerallythe most effective in resisting damage from wear and from very intense laser sources. The inorcianiccolourants in glass are quite stable. Unfortunately, not ali absorbing filters can be readily casehardened to provide impact resistance, and clear plastic sheets have often been placed with the filter.Absorbing-type filters are not always available which have sharp transmission "cut-off" near laserwavelengths in the visiblf.

c. h•ne advantage of using reflective coatings is that they can be designed with relatively sharpspectral "cut-offs" to selectively reflect a given wavelength while transmittinq as much of the rest ofthe visible as poscible. However, some angular dependence of the spectral attenuation factor is generallypresent. Hence, dichroic coatings are used generally only in conjunction with absorbinq filters, or insmall field-of-view optical instruments. At present, I think this typ/e of filter is most desirable forbinoculars and telescopes. The advantages of using absorbing plastic filters materials are: greaterimpact resistance, lighter weight, and ease of molding into curx-ed shapes. The disadvantages are: theyare more readily scratched, quality control appears to be more difficult, and the orqanic dyes used asabsorbers are more readily affected by heat and ultraviolet radiation and may saturate or bleach underq-switched laser irradiation. In my laboratory we encountered a number of plastic filters that undertoreversible bleaching. For example, a blue plastic filter had an optical density of 6.0 for CW 694.3 nmradiatior, but only had a density of 3 for a 30 ns laser pulse. Most of these problems have been solvedfor the plastic laser eye protection that is now commercially available, and one need only worry that someplastics will beccme denser with age causing the visual transmittance to be reduced.

9. SELECTING APPROPRIATE EYEMEAR. I like to follow a stop-by-step method for selecting eye protection.

STEP I. Determine Wavelength(s) of Laser Output

STEP II. Determine Required Optical Density. Table 1 lists required op.ical densitites (oralternatively, dB of attenuation, or attenuation factors) for various laser be&. int-nsities which couldbe incident upon safety eyewear. To determine the maximum incident beam intensity, consider thefolln--ing:

a. If the emergent beam is not focused down to a smaller spot, and is greater than 7-rn in diameter,A the emergenat beam radiant exposure/irradiance may be considered the maxinur'. that could reach the

unprotected eye, and .'s thus used in Table 1.

b. If the emergent beam is focused or viewed throuqh a telescope system or if the energent beandiameter is less than 7-cm in diameter, one should assumc that all of the beam energy,'o~wer could enterthe eye. In this case, you divide the laser output energy/power by the maxiniu area of the nupil(approximately 0.4 cm2 ). This equivalent radiant exposure or irradiance ray be used in Table 1.

c. If the observer is. in a vosition where he cannot receive ,.maximum outo,:t radiantexposure/irradiancze, then a measured value may be used. This is typFical for personnel "downranae" fromthe laser beam.

d. in general, having an opticdl density in excess of one density unit above ti-e riininmir reoruircementis not desirable since the visual tra.'srittance may be sacrificed. Additionally. it nay be realtired toview the laser beam of a C*W laser for alighniment, e.g. a 1 wat. argon laser could be safely worked withusing goggles with only a density of 3.5 to 4 which would -e-nit 5cmertary viewinq of the direct beei' --

although intentional direct viewing is not advisable.

STEP III. Determine Filter Damage Possiblity. If the maximum puilsed radiant exposure to the eyeprotection filter or frame exceeds lj-fcs-2 then dazage to the coggle could occur. Glass filters are most

desirable for protection against suhexposures. At these levels skin r~ratection --italso be developed.

STEP WV. Visual TIransmittance. Poor visual transmittance and reduced colour contrast as well asreduced peripheral v:ision in some goggles must be weighed against the benefits of the coggle. in com~batenvironments, the added risks of wearing many types of eye protection are too great tG warranL their useunless a very high probability of exposure to the direct bean exists.

10. CaUEKA SOURCES OF LASER EYE PROTECTI0I. At present no standard anti-laser aouqle for the U~Smilitary serv..ces has been produced. How-ever, a variety of cormercially available eye protection exists.Table !I prese-nts the optical densities at principal laser wavelengths and fox actinic ultravioletradiation (0.2 - 0.32 _=m) for several ty~pes of conmnercial eye proteztion of which I an most familiar inthe United States.

11. NhTIG LASER EYE PROTECTION. Eye prote,: 4--1 should be_ ch cked periodically for in.,aritv. Themeasurement of eye-protection-filter opetical tnsi ies in excess of 3 or 4 without destruction of thefiltur is v.ezy d'ifficult". Because of this problem, requiremnents orig:.-ally. prr-oosed for nany lase'-1.azard control guidelines, that the optical density of protective eyewear be periodical y, ch-ezcke-, avebeen deleted. The greatest concern has been with goqgles L~avin,? sp~ecf ied optical densitzes at, or onlyslightly above the density required for protection. Normally, requ7.ired densities do not. exý.'ed S.c3oggies having densities less than 8 are nor.a~lly desi;ned for use- at either the helixi-.eon o:- r-bbv laser

* ~wavelengths. Therefore, if a nore comprehesv gogle testing ' roorar er i itiated, the gcoocleswhcshould4 receive first attention are those hav--ng a density. less than 6 for the ri by a .nel tc-enlasers.Xv associates and I have periodically checked the optica; density of variois types of conmnerzia! eyeprotection. In general, the gogglts met or exceed specifica ions aiven b-y the :-anj~a:.:-rer a: ý4listed inTable II. However, in some rare instances protection filter- were shown to have ,?nit. 'ess thanspecified. In one case, t~he lower dens:tv sti.. exceeded 8 and was tht-refor.: ,ot of ::onceret. in a se-or.dcase, the dansity was significantly less than a specified density of 6. At present, all evidepzeindicates that the optical density of commercially available eycwt'sr does not decrtase witV. vse althL-uch

P some plastics become sl.,zhtly more dense after considerable exposire to solar radiation. and due t-, azinq.The actual measurement cf filter ontical densities between 3 a..d 10, and- n.erhavs qieate-r densities, can beeperformed with special techniques usina t.ather a spectre)meter or laser. Ps noted previously.

*spuztrophotometers found in nost chemical laborz-tories ar,ý linizreý lo neie.arzrients of densities 2 to 3.This l'.=_tation arises from difficIlties from "stray lv-ht" aesiro. t*'ro6.jh tht :,vonochrvr.mt-r'-_ Stravlight arises principally from dust and racrozicorpic xnperfect ions !r. the nrsn r iiffraction gratin~gs

hihscatter light of wavelengths other than the 1 w.~,ele:c=th of inter!;t. -;vioi~s'-, this stray lvobOcan be greatly reduced by placing monochrome,ýters in tandsiem or by usi:.- n;,r ow-Land -ilters (see Fi-ure b).Howev.er, measurement problems arise after one achieves a far n'rr cure monochrornatic bewan -f the detectordoes not have sufficient sensitivity. Vnles!c a laser is used as the light source, the brdit(slitwidth) of the monoobrometer ray be vc increased t-azhieve a measurahle OJO:.a. a, the det-_ct,-r that abraod-band attenuation factor is -reasured fo~r the protective filtei--a very serious shortcoming forfilters having a rapidly changing optical df-nsity with changirc wavelencoth. The use of lasers tr measurefilter transmission is iinfortunately limited to the wavele.,aths of the lasers ava:lw-.le. 1the las.:'- methodalso required the ube of narrow-band (laser "spike"! filters (Figure 5) ro elininate puwir li=1ht from.optically pumaped lasers or the glow dischzrge fron gas lasers. 'veasurement error-; can arise if the lase-

* output is not uniformly stable. One advantage of usir.q a )-switched ( -25 -is pulse) cr -.ode lock~ed (10-19*pr) pu:lsed laser is that reversible bleaching that can occur. -particularly in organILc dyes .:ted in pla!tic

filters may be detected.)5

12. M.ARKING OF Elf, PROTECTION. The optical density at .p~pi elaser wavelengths rhiould be m~rkedl onthe eye protection, since tho ust of goggles designet: fcz nor laser have bw.:sn r.± akenly used with anotherlaser a-nd could have rosulted in ocular injury. Less technicil -.erkinq, for cnam.,le 'u-si onli with ruaby1-ser", may also be desirable for field equipment.

11-8

TABLE 1. Selecting Laser Safety Glas

Non-Q-switchcd 0-switchcd

CW last- lascr lascr Attcnuation

Na1rnur Ma trnum MaximnumnstsS(dance irnad:ancc output .aummum Ma'imum Maximum Maximua m (4B Attenu.( .ci-:c l W- -: pocr (%V) rathint OupIut radaant output attenu- at;on

4 iomn.irarv1 corunuon ( continuous 2%jmoIurc .ne:t% t'iw-sur- energy 01) ation factorSl, ue st.,r~ngI b .tarinp 1 (J- 41 7) (J) (MI-cn :) (J)

1-4 '0 It" I0" 4 X l"' lir' X III I) 1 4 X I41-6 1 10 I(

4 X 10-2 10-4 4 X I'"- lit-" 4 x 1 l 10-6 4 X iO-- 2 20 I0:4 X l#-' I0-: t x ip-, III I I x lis- I0": 4 X 10-1 3, .1 0,4 to"! t x too-, III- I x 111r1 1ll-5 4 X 10-:' "1 40 10140 40 X t)-: 111- 4 X II' I Ii1-' I X 10-4 44) 50;

I go 4 X III-' 111 1 4 < too 1.0-: 4 X 51l13 4; 50) I(1

44, 04 Iti 4 Iw •o IpI' 4 X 10-: 7 71l 11Jt11 411 1 it 4 X to' I: III Ii"

I (5 lie

so.=.~ ~ 11-• l

0 .1* 44 2' C2 -2 422 fl 442S.4m- ,__lU2 .4_.1 - ____ , , t....••... .___..__ ._ .•_ •t':, .

w. . . . . . .. ... .,.. ... - 5". " 4-* A '," , , 4 ... 3 41 f? :.2 24 .4 , 142 .. .... I_"-: ! ":"' 44 .. .. . 2 4'2 1 22" : 41"" " " " 1.-' 2 .. .."

4 . 2, . , •. 2 44 , . 4 2 1,.24 42 .,4 .240. 42..2 .1 4 4 1 2 7. , 2

"*24 .. . . ', : 4: • : ' :23.4 4.I24"4242:) ; :" ". .

"".2, ..... 2 2 ..222 . .21.. 22 : . 4 24 .4 :2 .. .. 2 : 2 .2 ,22 '1..5 . r 2 . 5 . .2-, .. . 4. .. .. 22) .2 .2 .2.2 2.2 2 .. ; . " ! .

w-ia-- c-c c

- 4 . 2. . 2 . ... 2 ' 2 4. 1 4 . 2 .. 2... . . . .42 ..

'• •I •,'" :.V 2

" -. -' .. ... .2.22:: ,-. .. . .. ... • •. . ~ .

.2.224n l 1 I - I -l2i2i- l i-ii" ... • * 2 .2l1.2-i -5 2 2

-'9.

ADUAL BEAM -A. rSPECTROPHOTOMETER

ND FILTER.,In" 1. LINE FILTERS

B. U- 'CB.~ LAMP CHROMATERU

filN D FILTER SAFETY FILTER

C. LASER

fuFIGUj• 6. Three methods used for direct measurement of laser filter transmission are shown; several other

variations of combinations of filters light sources and monochromators are possible. In allarrangements, one or more neutral density filters calibrat-ad at the wavelength of interest areinserted in place of the protection filter until the transmission comparison is within I ODunit. This procedure is necessary to reduce errors for detector non-linearity. Laser linefilte-s arp "ontin'i -,us-, eA in series -'.t tle aAi.ion o•z € 1 r t-*$, recuction inunwanted light of other wavelengths).

13. EYE PROTECTION FOR INFRARED LASERS.

a. Optical radiation at wavelengths greater than 1.4 cm is absorbed in the anterior proftion of theeye and does not reach the retina. Protection standards for both the eye and skin have normally been ofthe same value. The need for skin protection as well as eye protection is therefore, necessary toconsider. Protection of the eye is nonetheless of paramount importance, since an injury to the eye(specifically the corneal stroma in most instances) can result in total or partial blindness, whereas theskin burn from a comparable laser dose would heal without such a disability.

b. :-zst optical materials which are transparent in the visible spectrun (transparent plastics, glass,and cuartz) are essentially opaque at wavelengths greater than 4.8 ;m. ?%l of these materials aretherefore used for eye protection for CO (5=m) and for C02 (10.6 .m) laser radiation. Plastic gogqles arepreferred for prctection against a low-probability of exposure reflections fro.. CO2 lasers naving anoutput poer less than 100 W despite the fact that the plastic may burn. Quartz (e.g. AO M4odei 300) orheat-resiscance glass (e.g. Hadron Type 112-4) goggles worn in conjunction with face snielln and skinprotection have been used when high-power CO2 laser beamc cannot be enclo-_ed.

c. Eye protection at wavelengths less than 5 .an has become a problem at certain wavelengths where.ucite" ', Plexiglassl and lime glass do not have absorption bands. The better approach used in theradiometry laboratory for an all purpose filter for wavelengths greatev than 1.4 am is a water cell.Because of weight and other design problems the H20 filter has not been. considered practical eyeprotaction method outside of a laboratory window. Although water goggles have be-' made] 6 , morepractical, lightweight goggles may be fabraicated by usinq 3 to 5 no Schott KG-3= glass filters whichprovide an OD of 3 to 5 at the deuterium-fluoride laser wavelenqths of 2.7 - 3.0 Lm and a- the hydrogenfluoride laser wavelengths of 2.9 - 3.2 cm.

14. EYE PROTECTION FOR PUMP LANPS AND TUNAaLE WAVELNGTH LASERS.

a. Occasionally eye protection is necessary to work with exposed arc lamps used as optical pumpingsources for pulsed or CK lasers. Eye protection developed for welding is quite suitable for this purpose.Likewise, some dye lasers may be scanned over most of the visible spectrum, and welding goggles mayprovide the only solution to some viewing requirements.

b. Eye protection filters for welders were developed empirically; however, optical transmissioncharacteriutics are now standardized as "shades" and spcified for particular applications. 1 7 1 Althoughmaximum transmittances for ultraviolet aisd infrared radiation are specified for each snade, the visualtransmittance t v or visual optical density Dv defines the shade niumber SO

Se•- 7/3 D + 1 (5)v

WhereDv = -log Tv (6)

For instance, a filter with a visual attenuation factor of 1000 (i.e., D 3) has a shade number of 8.Electric arcs typically have luminances of the order of 104 to 105 cd-cma 2 and filter densities rangingfrom 4 to 5 corresponding to shades 10 to 13 are required for comfortable viewing. 1 9 Likewise, a shade ofat least 13 is required to view the sun which has a luminance of approximately 105 cd cm-2 . Thesedensities are far in excess of those necessary to prevent retinal burns, but are required to reduce theluminance to I cd-cm- 2 or less for viewing comfort. The user of the eye protection should therefore bepermitted to choose the shade most desirable to him for his particular cperation. Actinic ultravioletradiation from quartz-enclosed arcs is effectively eliminated in all standard welding filters.

15. POLARIZING FILTERS. At first thought, the use of polarizing spectacles appears appealing as eyeprotection for multiple-wavelength use, since many lower output beams are highly polarized.Unfortunately, optical densities above two can scarcely be auhieved, and a tilt of the heal would renderthe protection almost non-existent. Nevertheless, such filters are often useful in a rotatable mount atthe laser exit port as a means of reducing the output to a reasonable safe level for alignment purposes inmany labiratory arrangements. Rotatable, cross-polarizing filters have occasionally been mounted ineyeweac to work with variable light so-arces, but their use is limited since ccsnercially availablepolarizing sheet material is effective in, a limited band of wavelengths, and while densities up to atleast 2, may exist in the visible spectrum, potentially hazardous lcvels of near-infrared radiation couldpass through the filters.

16. DYNAMIC EYE PROTECTION DEVICES. Numerous dynamic systems have been studied a. eye protection againstpulsed optical sources such as the nuclear fireball. The ideal dynamic filter is nearly transparentexcept when activated by a hazardous light source, at which time it rapidly becomes nearly opaque for theduration of the light flash. These systems usually consist of photc-detector-actuated shutters (which maybe mechanical, electro-optic, or magneto-optic) or photoreactive filters (such as photochromic materials).These devices are generally rather cumbersome when compared wich typical laser safety goggles or welder'sgoggles. Dynamic filters generally offer the only practical solution for eye protection againstunexpected white-light (broad-band) pulsed sources; however, this appraoch has not been require in thedevelopmsent of laser eyo protection, since sharp cutoff filters which attenuate the laser wavelength alsotransmit sufficient light for vision. Additionally, dynanic filter devices are not presently capable ofachieving significant optical densities even within 10 us which is far greater than the duration oftypical q-switched laser pulse (".20 ns), although such a fast response is theoretically possible. 2 0- 2 5

Image converter viewers designed to view near-infrared radiation and as night-vision viewing devices canserve as Isa er protective eyeivear. Although the image converter tubes may be damaged by direct laserirradiation, these devices provide equivalent optical densities of at least 8 for all wavelengths. Theirdisadvantare is bulkiness and monoc)romatic presentation with some loss of resolution of the objects beingobserved.2

17. FUTJURE DEVELOPmENTS.

a. It is difficult to predict future developments in laser technology. In the future, more lasersystems will be available in the infrared, and improvements may be expected in infrared detectors. It

appears reasonable that present laser applications which require unenclosed lasers (e.g. laser distancemeasurement equipment), but do not require a visible beam (as do alignment lasers) may best be realized byusing an infrared laser operating in the relatively "eye-safe" region beyond 1.4 um.

b. National safety codes being developed will probably encourage the manufacturing of enclosed lasersystems, and lasers which required the use of eye protection will probably be limited largely to themilitary and research environment.

c. It is technologically feasible that new filter materials could be developed which have narrowerabsorption bands in the vicinity of a laser wavelength. Such a development would be more likely to be ina plastic material rather than in a glass. Tntsrference filter coatings sealed between layers ofabsorbing plastic now show promise.

16. REFERENCES.

1. Schreibeis, W. J., 1968, Laser eyo protection gogt.les, based on manufacturer's information, Am.Industr. Hyg. Assn. J. 29:504.

2. Swope, C. H., 1969, The eye-protection, Arch. Environ. Health 18:428.

3. Swope, C. H., 1970, Design considerations for laser eye pzotection, Arch. Environ. Health 20:184.

4. Swope, C. H. and Koester, C. 3., 1965, Eye protection against lasers, AppI. (t. 4523.

5. Scherr, A. E., Tucker, R. j. and Greenwood, R. A., 1969, New plastics absorb at laser wavelengths,"Laser Focus 5:46.

6. Straub, H. W., 1965, Protection of the human eye from laser radiation, Ann. N.Y. Acad. Sci. 122:773.

7. Straub, H. W., 1970, Laser eye protection in the USA, Die Berufsgenossenschaft Zeitsnc-ift furUnfallversicherung und Betriebssicherheit 3:83.

8. Sliney, D. H., "Laser Protective Eyewear" in Lasers in Medicine and Biology pp.223-240, New YorezPlenum Press, 1974.

9. Sliney, D. H., 1970, Evaluating health hazards from military lasers, JA.IA 214:1047.

10. Ditchburn, R. W., 1963, "Light", second edition, chapter 15, John Wiley and Sons, New York.

11. JENA Glaswerk Schott and Gen., 1962, "Color Filter Glass", p. 19, Schott and Gen., Mainz, WestGermany.

12. Commission Internationale de L'Eclairage, 1970, "International Lighting Vocabulary", 3rd ed., page51, Bureau Central de la CIE, Paris.

13. Bauer, G., Hubner, H. J., and Sutter, E., 1968, Measurement oZ liqht scattered by eye protection

filters, Appl. Opt. 7:325.

14. Cook, R. B. and Jankow, R., 1973, Effects of stray light in spectroscopy, J. Chem. Educ. 49:405.

15. Holst, G.

16. Spencer, D. 3. and Bixler, H. A., 1972, IR laser radiation eye protector, Rev. Sci. Instr. 43:1545.

17. American National Standards Institute, 1959, "Safety Code for Head, Eye, and Respiratory Protection",V.S Z-2.1, American National Standards Institute, New York.

18. Coblentz, W. W. and Stair, R., 1930, "Correlation of the sh-.4- -. obers and densities ofeye-protective glasses", NBS Circi' .r 471, National Bureau fo St . . Washington, DC (November 1930).

19. Sliney, 0. H. and rreasier, B. C., 1973, The evaluation of optical radiation hazards, Appl. Opt.12:1.

2D. Fox, R. E., 1961, "Development c& Photoreactive Materials for Eye-Protective Devices", Report No.61-67, USAF School of Aer.ospace Medicine, Brooks Air Force Base, Texas 'AD 261608).

21. Harris, J. 0., Jr. ax:dl Cutcheln, J. T., 1972, Electooptic Variable Density Optical Filter, SandiaLaboratories, Albuquerque, .'.M.

22. Thursby, W. R., Richey, E. 0., Bartholomew, R. V. and Ebbers, R. W., 1971, "Evaluation ofPhotochromic Goggle System for Nuclear Flash Protection", SA%O-TR-71-20, USAF School of Aerospace Medicine,Brooks Air Force Base, Texas (AD 726544).

23. Williams, D. W. and Duggar, B. C., 1965, "Review of Research of Flash Blindness, Chorioretine.l Burns,Countermeasures, and Related Topics," DASA-1576 rev., Defense Atomic Support Agency, Washington, DC.

S4

B-I

BIBLIOGRAPHY ON LA-R ILWARDS AND SAEYIN TiE MILITARY EVI•INWMT

Compiled by

R.H. Oseman

Defence Research Information Centre

Procurement Executive, Ministry of Defence, U.K.

in collaboration with

R.G. Borland

RAF Institute of Aviation •I•dicineFarnboroughHampshire, U.K.

4

CONTENTS

Page

Introduction ....................................................... I

Sources and Availability of References Listed ...................... I

Guide to RECON Report Citations ..................................... I

Bibliography

General Review Papers ........................................... 3

Ocular Hazard Research .......................................... 4

Safety and Hazard Analysis ...................................... 7

Eye Protection .................................................. 8

Author Index ....................................................... 1O

r-

INTRODUCTION

This bibliography has been compiled by the Defenre Research Information Centre to provide literaturereferences to the problemb of hazards and safety in the use of lasers in support of the AGARD LectureSeries No. 79 - "Laser Hazards and Safety in the Military Envitonment". The programme of this lectureseries is related to laser radiations and the nature of ocular damage from- laser radiation, protectionagainst lasers, impli,ations of safety codes, the opthalmic examination of ldser uorker-;, and inveatigationof accidents.

The bibliography has been compiled using the ESRO RECON information network terminal at DRIC and is basedupon the .ASAISTAR-IAA file.

The references are of items announced in the period 1970-1974.

Abstracts have not been included, as the RECON system only supplies bibliographic details and descriptors.The citations are presented in reverse chronological order within each section. An author index, listingthe first two authors only, is included.

SOURCES AND AVAILABILITY OF REFERENCES LISTED:

Items of the type beginning N73-220468 were obtained from the NASA publication Scientific and TechnicalAerospace Reports (STAR) and copies of these reports are generally obtainable from national libraries orinformation centres, usually in microfiche form.

Items of the type beginning A73-24680 were obtained from the American Institute of Aeronautics andAstronautics publication International Aerospace Abstracts (IAA). These are of published literature andthe source is quoted in the reference.

GUIDE TO RECON REPORT CITATIONS:

A typical RECON citation is reproduced:

, 0 0 0 GAccession number Issue Subject category Report number Publication date

STARýr AW 1A

71:35528 ISSLU 22 CATEVORY 14 REPT-1204OEBYThP-00524 00/08/70 •UNCLASSIFIED REPORT

Title --- AERIAL PHOTOCRAPHIC TRACI;G OF PULP MILL. EFFLUENT

IN MARINE WATERS (AERIAL PHOTOGRAPh" FOR F ONI-TORING AND EVALUATING EFFLUENTS FROM1 OCEAN Notation of

Authors WASTE DISPOSAL PROCESSES)-] content"-"-- GLTRGESS, F.J. J&MES, W.P.

OREGON STATE UNIV. CORVALLIS. (OZ736722)Corporate DEPT. OF CIVIL ENGINELRING. 31 P. 14 REFS.AVAILSOD$1.25 -------. Availabilitysource / *AERIAL PHOTOGRAPHY / CO.-PLTER TECHNIQUES /

ONC-ENTRATION / DIFFUSION COEFFICIENT / *EFUF!y5/ A IO 4O.NITORS / *OCEANS / PAPERS D CI)TEPERATME DISTRIBUT-ION / TRANSPORT PROPERTIESI *WASTE. DISPOSAL / WATER POLLUTION

The above citation contains a large amount of useful information in a compressed form. The followingis given as a guide to understanding the various parts of thc. references in your printout. It shouldperhaps be noted that not all elements will be included in every reference.

I The ACCESSION NUMER is a unique number given to every document. By looking in the appropriateabstract journal (see below) under this number you will find a complete abstract of the document.

71 N 35528

year document entered actual numberfile (also year of of documentabstract journal)

N signifies the document is anunpublished report. Ii theletter is A then the documentis a journal article or conferenceproceeding.

N.B. A An asterisk (*) after the accession number indicates the document is a NASSA or NASA-sponsoredreport.

2 Depending on whether a document is a report (N) or a journal article (A) it is abstracted ineither the NASA STAR abstract journal (N) or the AIAA abstract journal (A). Each appears everyalternate fortnight - a total of 24 ISSUES per journal per year.

3 Within each ISSUE are 34 SUBJECT CATEGORIES. Thus in the example the accession number 71%35528will be found in CATEGORY 14 (Instrumentation and Photography) of ISSUE 22 of the 1971 NASA STAR abstract"journal.

N.B. - In the abstract journals from 1970 onw'ards - the format of the accession number is N71-35528.

4 LIn % documents (i.e. reports) there are very often one or more REPORT NUMBERS assigned by thecorporate source for internal and :dentification purposes.

5 The PUBLICATION DATE refers to the date of publicatiln of the document, in this case AUGUST 1970,not the date the document entered the file. The publication date should not always be taken as a truei=dication of Lhe date of issue. More often it relates to the date the report number is assigned ardthe real date of publication or issue may be several months later.

6 T1he NOTATION OF CON'TENT (NOC) contained in brackets af.er th-_ title is a microabstract giving alittle more infor--ation about the document. It is the NOC which appears under the descriptors andcorporate sources in the abstract journal indexes.

7 Each document is assigned several DESCRIPTORS which at'equately describe its contents and which areused for subsequent retrieval of the document. These descriptors can give you supplementary informationon the usefulness of the report. Those descriptors prefixed by an asterisk (*) reflect the mostimportant concepts and are the terms appearing in the subject index of the abstract journals.

".3

General Review Papers

1 •74A19542 ISSUE 07 CATEGORY 16 000073APPLICATIONS OF THE LASERS --- BOOKGoldnan, L.

Cleveland, CRC Press, Inc., 1973. 315 p. AA(Cincinnati, Univ:!rsity, Cincinnati, Ohio) REFS.522 *26*BIOTECNOLOGY/ CHEMICAL LASERS/ DENTISTRY/ DIAGNOSIS/ PO'l'IIELF I.o-UTION/ *HOLOGRAPHY/

LASER MATERIALS/ *LASERS/ *.MEDICAL EQUIPMIAT/ METAL WORKING- "'!I i ;i TECHNOLOGY/ *OPTICAL_F- CO.MMUNICATION/ PHOTOGRAPHIC RECORDING/ RADIATION HAZARDS/ -. !a ACTORS/ *TECHLIOLOGY

UTILIZATION/ THIN FILMS

2 72A21333 ISSUE 08 CATEGORY 05 000971THE BIOEFFECTS OF LIGHT. (BIOLOGICAL HAZARDS OF HIGH INTENSITY LIGHT SOFrCES, CONSIDERINGPHYSIOLOGICAL FACTORS INVOLVED IN THRESHOI.D EYE DACAGE VALUES DETRINA.rlON.)

Van Pelt, W.F.; Pavr.e, W.R.; Stewart, H.F.; Peterson, R.W.

Optical Spectra, Vol. 5, Sept. 1971. p. 33-36. AD(U.S. Department of Health, Education, andWelfare, Food and Drug Administration, Washington, D.C.) REFS. 8*BIOLOGICAL FFFECTS/ *EYE (AAITO.MY)/ INFRARED RADIATION/ LASER OUTr'bTS/ *LIGHT SOURCES/*LUMINOUS I.NTENSITY/ tRADIATION DAMAGE/ TISSUES (BIOLOGY)/ ULTRAVIOLET RADIATION

3 72A17945 ISSUE 06 CATEGORY 16 000071HA"DBOOK OF LASERS WITH SELEC2ED DATA ON OPTICAL TECHNOLOGY. (HANDBOOK ON LASERS AND OPTICALTECHNOLOGY COVERING GAS, DIE. LIQUID, INJECTION AND INSULATING CRYSTAL LASEkS, MATERIALS, SOURCES,TRANSMIFSION, HAZARDS AND HOLOGRAPHIC RECORDING)Pressley, R.J.Cleveland, Chenical Rubber Co., 1971. 630 p. AA(Holobeam, Inc., Paramus, N.J.) *27.50*CHEXICAL LASERS/ EYE PROTECTION/ *GAS LASERS/ HiANDBOOKS/ HOLOGRAPHY/ *INJECTION LASERS/*LASER MATERIALS/ LIGHT SOURCFS/ LIGHT TRANSMISSION/ *LIQUID LASERS/ OPTICAL DATA PROCESSING/*OPTICAL PROPERTIES! RADIATION HAZARDS/ TABLES (DATA)

7LA42426 ISSUE 22 CATEGORY 16 000071EFFECTS OF Hl.-lH-POWER LASER RADIATION (BOOK ON HIGH POWER LASER RADIATION COVERING HEATING, MELTING,VAPORIZATION, PARTICLE EMISSION, PLASMA PRODUCTION, GAS AND TRANSPARENT MATERIAL BREAKDOWN AND

BIOLOGICAL EFFECTS)Ready, J.F.AA/Hone.%-;:ll Corporate Research Center, Hopkin-;, Minn./. 438 p. New York, Academic Press, Inc.,

DOL. 17.50.*BIOLOGICAL EFFECTS/ GAS DISSOCIATION• *LASER HEATING/ LIGHT BEAMS/ *PARTICLE EMISSION/#.FLASMA GENERATORS/ *RADIATION EFFECTS/ TRANSPARENCE/ VAPORIZING

71A'1795 ISSUE 22 CATEGORY 14 000771INSTRL-.NTATI)N AND HEASLREM.N'I OF ULTRAVIOLET, VISIBLE, AND INFRARED RADIATION (HIGH INTENSITY

LIGHT SO1XCES HAZARDS ANALYSIS, DISCUSSING THERMAL DETECTORS AND VACUUM AND SEMICONDUCTOR PHOTO-DIODES FOR PCLSED LASER OUTPUTS TEASUREE•N.%T)Bason, F.C.; Freasier, B.C.; Sliney, D.L.American Industrial •iygiezie Association Journal, Vol. 32, P. 415-431. AB/U.S. Ar-y EnvironmentaiHygiene Agency, Edgewood Arsenal, MD./. REFS. 2331OLOGICAL ETFECTS/ ELECTRON ENERGY/ INFRARED RADIATION/ *LASER OU*TPLTS/ *LL.4INOUS INTENSITY/*PHOTODIODLS/ *PULSED LASERS/ *RADIATION HAZARDS/ *SEMICONDUCTOR DEVICES/ ULTRAVIOLi'T RADIATION

6 71A16481 ISSUE 05 CATEGORY 05 000070PROGRESS IN OPTICS. VOLUM.E 8 (PAPERS ON OPTICS, VOLUME 8, COVERING SYNTHETIC APERTURES, LIGHTBEATING SPECTR.,COPY, NIULTILAYER ANTIREFLECTION COATINGS, INTERFERENCE MICROSCOPY, PHOTOELZCTRONCOUNTING, HLMAN EYE PERrORH.A.CE, LASER LIGHT, ETC)Wolf, E.Plac- Amsterdam, Publ- North.-Hollcnd Publishing Co., Date- 1970. Coll- 467 P. $21.APERTURES/ BEAT FREQUENCIES/ COATINGS/ COHERENT LIGHT/ DIFFRACTION/ EYE (ANATOMY)/ HLU-MA

PERFORMANCE/ I:TERFERENCE/ LASER OUTPUTS/ *MICROSCOPY/ *OPTICS/ *PHOTOELECTRONS/ *SPECTROSCOPY/

*SYNCHRONISM

7 70A21043 ISSUE 08 CATEGOhY 05 DA-49-193-MD-2241, DA-49-146-XZ-416 000270THE EYE PROBLEM1 IN LASER SAFETY (BIOLOGICAL EFFECTS OF LASER RADIATION ON HUMAN EYE, DISCUSSINGDAMAGE CAUSED BY LONG TFRM EXPOSURE TO VISIBLE, ; IR AND ; Ue WAVEI.ENGTHS)Clarke, A.M.; Cleary, S.F.; Geeraets, W.J.; IL-m, W.T.; .Jr.; Nueller, H.A.; Williams, R.C.Conf- /International Laser S.fety Conference and Workshops, 2nd, Cincinnati, Ohio, Mar. 24, 25, 1969./

Init- Archives of rnviroraental Health, Vol. 20, P. 156-160. Coil- 12 REFS. Date- Feb. 1970.*BIOLO.ICAL EFFECTS/ CONFERENCFS/ *EYEE (ANATOMY)/ INFRARED RADIATION! *LASER OUTPUTS/ LIGHT

(VISIBLE RADIATION)! *RADIATIOS DAMAGE/ *•DIATION DOSAGE/ SAFETY FACTORS/ ULTRAVIOLET RADIATION

0.4

Ocular Hazard Research

8 74A364141 ISSUE 17 CATEGORY 05 DADAl7-72-C-21/7 260774OCULAR iHAZARD FROM PICOSECOND PULSES OF Nd:YAG LASER RADIATIONHam. W.T.. Jr.; Mueller, Ht.A.; Goldman, A.I.; Neenmm, D.E.; Holland, L.M.; Kuwabara, T.Science. Vol. 185, July 26, 1974, p. 362. 363, Research supported by University of California;AC(Virginia Commonwealth University, Richmond, Va.) AE(California, University, Los Alaaos, N. Mex.)AF(National Institutes of Health, National Eye Institute, Bethesda, Nd.) lefs. 10 Jpn. 2385ELECTRON MICROSCOPES/ *EYE EXAMINATIONS/ LASER INODE LOCKING/ *LASER ObLTPUTSI NDNKEYS/.EODYMIL./ *PULSE DURATION/ *PULSED LASERS/ *RADIATION INJURIES/ RETINA/ 'TIRESHOLDS(PERCEPTION) / YAG LASERS

9 74N16837' ISSUE 08 CATEGORY 04 AD-770404 FA-M73-25-1 DA PROJ.ITO-61102-A-31C 000873

'C NEODYMIU.' OCULA? DAMAGE THRESHOLD STUDY. ONE-SECOND EXPOSURE DURATION/INTERIM REPORT/Lund, D.J.; Carver, C.T.; Zwicker, W.E.Frankford Arsenal, Philadelphia, Pa. (F4331335) 16 P. Jpn. 883CUMULATIVE DAMAGE/ *EYE (ANATOMY)/ *LASERS/ MNOKEYS/ 'NEODYMIUM/ *RADIATION INJURIES/RETINA/ THRESHOLD DETECTORS (DOSIMETERS)

10 74N286259 ISSUE IS CATEGORY 0. AD-777144 SAM-TIR-74-1 AF PROJ. 6301 000274OCULAR DAMAGE THRESHOLDS FOR REPETITIVE PULSED ARGON LASER EXPOSURE/INTERIM REPORT, Sep. 1972-Sep. 1973/Gibbons, W.D.; Egbert, D.E.School of Aerospace Medicine, Brook- AFB, Tex. (0D261436) 2i P. Jpn. 2137*ARGON LASERS/ *EYE (ANATOMY)/ HEALTH PHYSICS/ NONKEYFI PULSED LASERS/ RADIATION DOSAGE/

RADIATION HAZARDS/ 'RADIATION INJURIES% RADIOBIOLWGY/ *THRESHOLDS

11 74N168351 ISSUE 08 CATEGORY 04 AD-770561 SAM4-TR-73-45 AF PROJ. 6301 001173RETINAL BUR.% THRESHOLDS FOR EXPOSURE TO A FREQUENCY DOUBLED hTODYK'UM LASER/FINAL REPORT,4 Apr. - 5 Jun. 1973/Gibbons, W.D.School of Aerospace Yedicine, Brooks AFB, Tex. (SD261436) 16 P. Jpn. 888AEROSPACE .EDICINE. 'BURNS (INJVRIES)/ EYE (ANATOMY)/ 'LASER OUTPUTS/ 'WNKETS/ WBU•MTMIPHYSIOLOGICAL EFFECTS/ RADIATION HAZARDS/ *RADIATION INJURIES/ PADIOBIOLOY/ 'RETINA/THRESHOLDS

12 73A25341 ISSUE 11 CATEGORY 04 000373RETINAL DAMAGE THRESHOLDS FOR MULTIPLE PULSE LASERSEbbcrs. R.W.; Dunsky, I.L.Aerospace Medicine Vol. 44, Mar. 1913, p. 3;7, 31U. AR(USr, School of Aerospace Medicine,Brooks AFB, Tex.) REFS. 10 Jpn. 1315BIOMETRICS/ *EYE PROTECTION/ .HCSEYS/ NECDYMTIEX/ IULVE AIt'LITUDE/ PULSE DUtATIONCt'PULSED LASERS/ Q SWITCHED LASERS/ *RADIATION DAM.AGE! .ADIATION INJURIES!RAD)IATION TOLERA.NCE/ RETIN!/ SEMICONDUCTOR LASERS

13 72A35396 ISSUE 17 CATEGORY 05 070772RETINA - ULTRASTRUCTURAL ALTERATIONS PRODUCED BY EXTREMELY LOW LEVELS OF COHERENT RADIATION.(RHESUS MON.EY RETINAS ULTRASTRUCTURAL ALTERATION AND DAMAGE IN RODS AND CONES PRODUCED BY QSWITCHED RUBY LASER COHEREN'T RADIATION)Adams, D.O.; Beatrice. E.S.; Bedell, R.B.Science, Vol. 177 July 7, 1972, p. 56-60. AC(U.S. Army, Joint Army Medical Research andDevelcrament Command, Frankford Arsenal, Philadelphia, PA.) REFS. 16 Jpn. 2509'COHERE•T RADIATION/ LASER OUTPUTS/ MONKEYS! I'ATHOGENESISI *PHYSI*LOGICAL TESTS/ PULSEDRADIATION/ Q SWITCHED LASERS/ RADIA1' FLUX D.NSITY/ *'RADIATION DIMAGi/ 'RETINA/ RUSY IASERS

14 73N13096' ISSUE O4 CATEGORY 04 AD-746795 F•!.•O9-71-C-OO18 300672OCULAR EFFECTS OF REPETITIVE LASER PULSES/FLNAL REPORT, Apr. 1971-Jun. 1972/ (ARGON-ION LASERUSED TO DETERMINE OCULAR EFFECTS OF REPETITIVE LASER PULSES ON RHESUS MOIKEYS)Skeen, C.H.; Bruce, W.R.; Tips,J.H.,Jr.; Smith. M.G.; Garza, G.G.Technology, Inc., San antonio, Tex. (TJ954426)Life Sciences Div. 93 p. Jpn. 380'GAS LASERS! MONKY1S! PULSED RADIATION! *RADIATION INJURIES/ RADIATION TOLERANCE! 'RETINA/TABLES (DATA)

8.5

Ocular Hazard Research

15 74N127750 ISSUE 04 CATEGORY 04 AD-766255 TlR-074(4240-10)-4 SA(SO-TI-73-215 F0701-73-C-0074,1507731tRELIM•IUAY CORNEAL DO = THRESHOLD STUDIES WITH HF-DF CHIEICAL LASERS/TECHNICAL REPORT,Feb.-Mar.1972/Spencer. D.J.; Dunsky, I.L.Aerospace Corp., El Segundo, Calif. (AG:63093) 26 p. Jpn. 365*CREICAL LASERS/ ACOKEAl DEU+ERIUt) FLUORIDES/ HYDROGEN/ LASER OUJTPUTS/ MOtE-YSl"MAIATION INJURIES/ WAVELENGTHS

16 73N20135# ISSUE 11 CATEGORY 04 AD-753419 SA-E-TR-72-23 001172RLETINAL EFFECIS OF MULTIPLE PULSE GALLIUM ARtSENIDE LASER/IFNAL REPORT. 1971/ (RETINAL DAMAGE IN

M)NO1YS EXPOSED NEAR INFRARED GALLIUM ARSENIDE LASER PLSES)Ebbers, L..School of Aerospace Medicine, Brooks AFB, Tex. (SD261436)17 p. Jpn. 1248GALLIUM ARSENIDE LASERS/ *IhtRARED LASERS/ *LASER OUTPUTS/ 8ONK.EYS/ PULSED lASERS/ RADIATIONDOSAQ/ "WADIATION IKJURIES/ RAVeIOBIOLOGY/ 'RETINA

17 72A12413 ISSUE 02 CATEGORY 04 191171THRESHOLD LEVELS FOR DAMAGE OF THE CORNEA FOLLOWING IRRADIATION BY A CONTIN•OUS WAVE CAfl.ONDIOXIDE (RABBIT AN MOM•-Y CO•EAL DAMAGE FOLLOWING Ca CARBON DIOXIDE LASER IRRADIATION,DISCUSSING HAZARD LEVEL DERIVATION)Borland, I.G.i Brennan, D.H.; Nicholson, A.N.Nature, Vol. 234, Nov. 19, 1971, p. 151, 152. AC(RAF, Institu•e of Aviation Medicine. Farnborough,Hants., England)#ICAUBON DIOXIDE LASERS/ 'CONTINUOUS RADIA/IONI *CORNA *LASER CUTPUTS/ MONKEYS/ RABBITS/RADIATION DA.'IAG/ *RADIATION HAZARDS" •'WRAITION INJUR"L.S

18 72.N15078* "SSUE 06 CATEGORY 04 AD-728852 EG/G-S-543-R DADAI7-69-C-9013 140771LASER EYE EFFECTS: THE SUBVISIBLE RETINAL LESION/FINAL REPORT, 1 Aug. 1969 - 31 Jul. 1971/(Z-MICT OF HELIU.f-NEON AV. YAG LASER RADIATION ON RETIW-' OF RABBITS AT LEVELS BELOW THOSE REQUIREDTO PRODUCE VISIBLE LESIONS)Mautner, W.J.Edgerton, Ger=esh;,sen and Crier, Inc., Goleta, Calif. (EE276221) 93 p. Jpn. 724COHERENT RADIATION/ EYE (ANATVOM)/ *LASER OUTPUTS/ *PHYSIOLOGICAL EFFECTS/ RABBITS/ *RADIATIONDA.MAGE/ 'RETINAl SENSITIVITY/ TISSUES (BIOLOGY)

19 71A38284 ISSUE '9 CATEGORY 05 000771EVALUATION OF RETINAL THRESHOLDS FOR; C.W. LASER RADIATION (3ETINAL DAMACE THRESHOLDS OF RHESUSNOMOI(YS TO OCULAR RADIATION FROM YELLOW LINE 568.2 M EMITTED BY KRiPTON; CW GAS LASER)Dunsky, I.L.; Lappin, P.W.Init- Vision Reseerch. Voi.l1. P.733-738. Coil- 8 Refs. Yul. 1971.

ARGON/ AGAS LASERS! HEL:LW-NEON LASE.S/ *KRYPTON/ *LIGHT (VISIBLE RADIATION)/ MONKEYS/NEODYMIIM/ 'RADIATIUN DAWAGE/ *RETINA/ YTTRILU-ALUHINIUM GARNET

20 71A32347 ISSUE 15 CATEGORY 05 000671OaJLAR EFFECTS OF ARGON LASER RADIATION. ;1I - HISTOPATHOLOGY OF CHOkIORETINAL LESIONS(HISIOPATHOLOGICAL AM FLUORESCEIN AIGIOA•PPHIC STUDIES OF RHESUS MONCEY CHORIORETINAL LESIONSPRODUCED AT THRESHOLD AND SUPRATHERESHOLD POWER LEVEL. OF AR LASER)Bresnick, G.H.; Chester, J.E. Frisch, J.E.; Frisch, G.D.; Powell, J.O.; Yanoff, M.Tnit- American Journal of Ophthalmology, Vol. 71, P. 1267-1276. Coil- 11 Refs. Jun. 1971.ELECTROMAGNSTIC ABSORPTION/ EPITHELIUM/ *EYE (ANATOMY)/ GAS LASERS/ HISTOLOGY/ *LASEROUIPUTS/ LESIO0SI N)ONKEYS/ PATHOLOGY/ PIGMENTS/ *RADIATION DAMAGE/ *RETINA/ TEMPERATUREEFFECTS

21 72N321270 ISSUE 23 CATEGORY 04 AD-741380 REPT-3 DADIW7-70-C-OOl1 00472HIISIOPAINGLOGY OF ARGON LASER-INDUCED RETINAL LESIONS/FINOL REPORT AUG. 1970 - 31 JAN. 1971/(RISTOPA.HOLOGY OF iAhGON, RUNY, GALLIUM ARSENIDE, NFODYNIUM, AND CARBON DIOXIDE LASER INDUCEDRETINAL LESIOUS)Y-norf, H.Pennsylvania Univ., Philadelphia. (PJ652i16) School of Medicine. 9 P. JPN. 3052SWLEMICAL LaSERS/ GAS LASEP.S/ HISTOLOGY/ *PATHOLOGY/ PHOTORECEPTOIS/ *RADIATION INJURIES/'RETINA 'RUBY LASERS

1140

Ocular Hazard Research

22 72.18535m ISSUE 09 CATEGORY 16 AD-731577 F41609-70-C-0002 S.11 PROJ. 8209 O0Ol7OCULAR LASER THRESHOLD INVESTIGATIONS/FINAL REPORT, 1 SEP. 1969 - 31 DEC. 1970!(THRKSHOLD LEVELS FOR DOUBLED NEODYMIUM AND RUBY LASERS)Vassiliadis, A.; Zi'eng. H.C.Stanford Research Inst.. "enlo Park, Calif. (SO132772) 66 P. JPN. 1217COHERENT RADIATION/ *NEODYMIU./ RADIATION EFFECTS/ RETINA/ *RUlBY LASERS/ *THIRESHOLDS(PERCEPTION)

23 71%14673, ISSUE 05 CATEGORY 04 AD-713511 000070RETINAL DAMAGE BY Q-SWITCHED RUBY LASER (RETI%,AL DAMAGE EY Q SWITCHED RUBY LASER)Beatrice. E.S.; Bresnick, G.H.Frankford Arsenal, Philadelphia. PA. (F4331335) Date- 1970 Coil- 15 P Refs Ajjil- NTISBLRNS (INJ'RIES)/ *Q SWITCHED LASERS/ *RADIATION INJURIES/ RADIATION TOLERANCE/*R5TINA/ *RUBY LASERS! TISSUES (BIOLOGY)

24. 7OA41997 ISSUE 21 CATEGORY 05 120970NE. OCULAR HAZARD OF MODE LOCKING IN : CW LASERS (THRESHOLD RETINAL DAM'AGE BY : CVHE- NE LASERS DýE TO MODE LOCKING)Manson, N.; Marshall. J.Init- Ncture, Vol. 227, P. 1149. 1150. Coll- 5 Refs. Date- Sep 12, 1970CONTINL•US RADIATION/ GAS LASERS/ *LASER MODES/ *LASER OUTPUTS/ LIGHT BEAMS! POWERGAIN/ *RADIATION DAMAGE/ *RETINA! THRESHOLDS

25 70N39661z ISSUE 22 CATEGORY 04 AD-707871 ARL-TR-70-9 F29600-69-C-0007 000570BEHAVIOURAL ASSESSMENT OF VISUAL FUNCTIONING IMMEIDIATELY AFTER EXPOSURE OF THE EYE TO ALASER (BEHAVIOURAL ASSESSMENT OF VISUAL FUNCTIONING IN RHESUS MONKEYS I'M1EDLTELYFOLL(OWIG LASER INDU:CED FOVEAL IMPAIRMENT)Farrer. D.N.; Fields, T.A.Washington State Univ., Pullman. (WFO43951)Plac- Hollozan AFB, N. Mex. Publ- ARL Date- May 1970 Coil- 21P Rei:s Avail- NTIS*BbU.NS (INJURItS)/ FLASH BLINDNESS! *LASERS/ *'0.NKEYS! *RETINA/ 'VISiIAL ACUITY/

VISUAl. PERCEPTION

26 71.%14474.5 ISSUE 04 CATEGORY 04 AD-702865 REPr-7005-703F F41609-68-C-0038 000270MEASURM.E•NHT OF RETINAL IMAGE FOR LASER RADIATIO. I RIESUS MO.NEY FNAL REPORT(.EASURE.ENT OF RETINAL IMAGE IN RHESUS MONMEYS TO DETERMINE LASER RADIATION HAZARDS)Elgin, S.S.; Stein, M.N.Eye Research Foundation of Bethesd, .M. (E990.016?Date- Feb. 1970 Coll- 37 P Refs Avail- NTISFIBER OPTICS/ *LASERS/ MONKýEYS/ *OPTICA.L MEASUREMENT/ PHOTOGRAPHIC RECORDING/'RADIATION HAZARDS/ PRETINAL IMAtES•i SYSTEMS ENVINEERING/ THRESIOLD DETECTORS

(DOSIMETERS)

27 70A21044 ISSUE 08 CATE.GORY 35 NIH 5-ROI-EY-00318-04, AF 3316151-3060, AF 33/615-67-C-1752,AF 41/609/-68-C-O041 000270THRESHOLDS OF LASER .YE HAZARDS (RETINAL DAMAGE THRESHOLDS BY EXPOSING RIESUS MONKEYAND HUMAN EYES TO LASER RADIArIO., TESTING RABBI7 EYES FOR CORNEAL TdRESHOLDS)Honey, R.C.; Peabody, R.Conf- /International Laser Safety Conference and Workshops, 2nd, Cincinnati, Ohio,Mar. 24, 25, 1969./ Init- Archives of Environmental Health, Vol. 20, P. 161-170.Co•I- 20 Refs. Date- Feb. 1970.CARBON DIOXIDE LASERS/ CONFERENCES/ CORNEA/ *EYE (ANATOMY)/ GAS LASERS/ 'JLASEROUTPULTS/ PULSED LASERS/ 'RADIATION DAMAGE/ RETINAL ADAPTATION/ RUBY LASERS/SAFETY FACTORS/ SOLID STATE LASERS/ *THRESHOLDS

28 7ON28315# ISSUE 14 CATEGORY 16 AD-700124, FA-M69-26-l 001069DETERMINATION OF VISIBLE THRESHOLD OF DAMAGE IN RETINA OF RHESUS MONKEY BY Q-SWITCHEDRUBY LASER (RUBY LASER THRESHOLD DAMAGE .ATA IN RETINAS OF rHESUS MONKEYS)Beatrice, E.S.; Byar, H.H.Frankfort' rsenal, Philadelphia, PA. (F4331335)Date- 0,--. 1969 Coll- 17 P Refs Avail- CFSTIMONKEY' Q SWITCHED LASERS/ *RADIATION INJURIES/ 'RADIATION TOLERANCE/ *RETINA!*RUBY L;ýiERS

_ 0-7

Ocular Hazard Research

29 70ON279208 ISSUE 14. CATEGORY 16 AD-700422 F41609-68-C-O041 000869RESEARCH ON OCULAR LASER THRESHOLDS FINAL REPORT. 15 MAR. 1968 - 15 JUL 1969(LASER THRESHOLD LEVELS FOR RHESUS MONKEYS WITH SHALL RETINAL LESIONS)Rosan, R.C.; Vassiliades, A.Stanford Research Inst., Menlo Park, Calif. (S0132772)Date- Aug. 1969 Coil- 88 P Refs Avail- CFSTI*EYE (ANATOMY)/ GAS LASERS/ LESIONS/ MONKEYS/ *Q SWITCHED LASERS/ RETINA!"*THRESHOLDS (PERCEPTION) / VISION

Safety and Hazard Analysis

30 73N27973* ISSUE 19 CATEGORY 05 AD-762277 SAM-TR--73-19 000673SHAZARD EVALUATION OF A GALLIUM ARSENIDE DIODE ARRAY LASER (RADIATION HAZARDS OF GALLIUM

ARSENIDE DIODE ARRAY LASERS)Gallagher, J.R.; Laudieri, P.C.School of Aerospace Medicine, tocoks AFB, Tex. (SD261436) 20 P. JPN. 2229EYE (ANIATOMYf) *cGp.1JI09 ARSLNIDE LASERS/ *RAD)IATION HAZARDS/ *RADIATION INJURlES!RADIOBIOLOGY

S31 72N.33121 ISSUE 24 CITEGORY 05 AD-744656 NADC-72041-AE 190572

EYE-SAFE OPERATION OF ILLUMINATOR-AIDED IMAGING SYSTEMS (EYE-SAVE LEVELS FOR OPERATINGILLUMINATED IMAGING SYSTEMS IN TERM14S OF MAXIMUM PERMISSIBLE CORNEAL IRRADIANCE)Campana, S.B.Naval Air Development Center, Warminster, Pa. (N0000154)A.oro-Electronic Technology Dept. 17 P. JPN. 3180CXR•TA! *DISPLAY DEVICES/ tEYE (ANATOIY)! GAS LASERS *IRRADIANCE/ RADIATION DAMAGE/RADIATION HAZARDS/ TELEVISION SYSTEMS

-!2 73N220144# ISSUE 13 CATEGORY 04 AD-755405 F41609-71-C-0029 AF PROJ. 7784 SRI PROJ. 1341000672AN I!VESTIGATION OF ATMOSPHERIC EFFECTS ON LASER PROPAGATION AND THE IMPACT ON EYE SAFETY/FIFALREPORT, JUN. 1971 - OCT. 1972/ (ATMOSPHERIC EFFECTS ON LASER PROPAGATION AND IMPACTON EYE SAFL'TY)Dabberdt, W.F.Stanford Research Inst., Menlo Park. Calif. (SO132772) 118 P. JPN. 1485'*AT5SPHERIZ ATTENUATION/ ATMOSPHERIC TURBULENCE/ *EYE (ANATOMY)/ *LASER OTP=UTS/OPERATIONAL HAZARDS/ *RADIATION EFFECTS/ RADIOBIOLOGY/ SCINTILLATION

33 71A345249 ISSJE 17 CATEGOR" 11 000571LASER SAFETY ON AN OUTDOOR RANGE *E!VIRO.%.NTAL CONTROLS, HEALIH SERVICES AND SAFETY PROGRAMS FOR

OUTDOOR RANGE LASER APPLICATIONS, CONSIDERING ;USAF HAZARD REGULATIONS, PUBLIC ADDRESS SYSTEM, ETC)Fallon, P.F.Conf- American Industrial Hygiene Assn., American Industrial Hygiene Conference, Toronto, Canada,May 24-28, 1971, Paper. Coll- 13 P. 7 Refs.CONFERENCES/ *'ENIRONMENTAL CONTROL/ *LASER MODES/ OBSERVATION AIRCRAFT/ PUBLIC ADDRESS SYSTEMS/*RADIATION HAZARDS/ *RANGE SAFETY/ REGULATIONS/ VISUAL OBSERVATION/ WARNING SYSTEMS

34 72N134569 ISSUE 04 CATEGORY 16 AD-729346 USAEHA-42-073-71 000471LASER DISTA7CE MEASURING EQUIPMENT% USED BY US ARMY TOPOGRAPHIC COM!(AND (USATOPO'_OM), -EBRUARY -A.PRIL 1971 (POTENTIAL HAZARDS OF FOUR TYPES OF LASER DISTANCE MEASLIING EQUIPMENT)Slincy, D.H.Army Environmental Hygiene Agency. Edgewood Arsenal, Md. (A0534823) 15 P. JPN. 493DISTANCE MEASURING EQUIPMENT/ *LASERS/ *RADIATION HAZARDS/ RADIATION MEASUREMENT/ SAFETY FACTORS

:"35 72N300730 ISSUE. 21 CATEGORY 04 AD-742267 SAM-TR-72-11 AF PROJ. 6301 000472DETERMINATION OF REVISED AIR FORCE PERMISSIBLE EXPOSURE LEVELS FOR LASER RADIATION/TECHNICAL REPORT,

JAN. - SEP. 1970/ (REVISED SAFE LASER RADIATION EXPOSURE LEVELS FOR AIR FORCE PERSONNL WORKING INVISIBLE AND NEAR INFRARED REGION)Dunsky, I.L.; Fife, W.A.School of Aerospace Medicine, Brooks AFB, Tex. (SD261436) 16 P. JPN. 2784*LASERS! *LIGHT (1ISIBLE RADIATION) *NEAR INFRARED RADIATION! OPERATORS (PERSONNEL)/ RADIATIONDOSAGE/ RADIATION HAZARDS/ *RADIATION TOLERA:SCE/ *SAFETY MANAGEMENT

B-8

Safety and Hazard Analysis

36 71N110740 ISSUE 02 CATEGORY 04 PB-189360 000470REGULATIONS, STANDARDS, AND GUIDES FOR MICROWAVES, ULTRAVIOLET RADIATION, AND RADIATION FROMLASERS AND TELEVISION RECEIVERS - AN ANNOTATED BIBLIOGRAPHY (ANNOTATED BIBLIOGRAPHY OFREGULATIONS, STANDARDS, AND GUIDES FOR MICROWAVES, AND ULTRAVIOLET, LASER, AND TELEVISIONRECEIVER RADIATION)Setter, L.R.; Snavely, D.R.Bureau of Radiological Real'h, Rockville, MD. (B8161222)Dat-- Apr. 1970 Coil- 84 P Refs Seri- Its PHS Publ. No. 999-RH-35 Avail- NTIS*BIBLIOGRAPHIES/ HAZARDS/ IIHALTH/ *LASER OUTPUTS/ *MICROWAVES/ *RADIATION HAZARDS/REGULATIONS! STA.DARDS! *TEI.EVISION RECEIVERS! *ULTRAVIOLET RADIATION/ X RAYS

37 70N186608 ISSUE 07 CATEGORY 04 AD-697151. BRL-).2-2012 000969PROBABILITY ANALYSIS OF OCULAR DAMAGE DUE TO LASER RADIATION THROUGH THE ATMOSPHERE(HATHEMATICAL MO)DEL FOR PROBABILITY OF OCULAR DAMAGE FROM PULSED LASER BEAN)Deitz. P.H.Ballistic Research Labs., Aberdeen Proving Ground, MD. (BCO32254)Date- Sep. 1969 Coll- 40 P Refs Avail- CFSTIAThOSPHERIC CIRCULATION/ BEA14S (RADIATION)/ EYE EXAMINATIONS/ LASERS/ *MATHEMATICALMODELS/ *PROBABILITY THEORY/ *RADIATION INJURIES

38 71A19791 ISSUE 07 CATEGORY 05 100270A CO.MENTARY ON LASER-INDUCED BIOLOGICAL EFFECTS AND PROTECTIVE MEASURES (CONTROL OF BIOLOGICALLASER RADIATION HAZARDS)Wilkening. G.M.Conf- /New York Academy of Sciences, Conference on the Laser, 2nd, New York, NY. May 2, 3. 1969/Init- New York Academy of Sciences, Annals, Vol. 168, P. 621-626. Coil- 34 Refs. Date- Feb. 10,1970BIOLOGICAL EFFECTS/ CONFERENCES/ *EYE PROTECTION/ *LASERS/ LIGHT BEAMS/ *RADIATION HAZARDS/SAFETY FACTORS! SKIN (ANATOMY)

Eye Protection

39 73N26099c ISSUE 17 CATEGORY 05 AD-759921 USAEHA-42-57-73 020573EVALUATION OF HADRON MODEL 112 LASER SAFETY EYESHIELDS, JANUARY - MARCH 19731/R'DIATION PROTECTIONSPECIAL STUDY/ (PERFORMANCE OF HADRON LASER SAFETY EYESH;ELDS IN PROTECTING PERSONNEL FROMACCIDENTAL OCLLAR E'POSURE TO COM.ON LASER RADIATION)Sliney, D.H.; Walkenback, J.E.Army Ewnironnental Hygiene Agency, Edgewood Arsenal, Md. (AO534823) 18 P. JPN 1991*ETYE PROTECTION/ *LASERS! OPTICAL DENSITY/ *PERFORMAJ.NCE/ RADIATION DOSAGE/ RADIATIONPROTECTION/ *SAFETY DEVICES

40 73N220579 ISSUE 13 CATEGORY 05 AD-755406 TR-619-F F41609-7!-C-0017 AF PROJ. 7784 001072GLASS OCULAR LASER PROTECTIVE FILTLRS/FINAL REPORT, 1 APR. 1971 - 1 AUG. 1972/ (OPTICAL DENSITYPROPERTIES OF OCULAR GLASS LASER PROTECTIVE FILTERS)Woodcock, R.F.A=erican Optical Co., Southbridge, 'lass (AS850297) Research Div. 35 P. JPN. 1487*EYE PROTECTION/ GLASS/ *LASERS/ *OPTICAL DENSITY/ *OPTIeAL FILTERS/ OPTICAL PROPERTIESSAFETY DEVICES

41 73N17127* ISSUE 08 CATEGORY 05 AD-752594 F41609-71-C-0019 AF PROJ. 7784 001072PLASTIC MATERIALS FOR EYE PROTECTION FROM LASERS/TECHNICAL REPORT, 3 MAY 1971 - 15 JUL. 1972/(PLASTIC .ATERIAJS FOR EYE PRCTECTION FROM LASERS)Sherr, A.E.; Cordes, W.F.Anerican Cyanamide Co., Bound Brook, N.J. (AQ581041) Organic Chemicals Div. 101 P. JPN. 880*EYE PROTECTION/ *GOGGLES/ *LASERS/ PLASTICS/ POLYMER PHYSICS/ PROPIONIC ACID/ STANNATES

42 73NI81420 ISSUE 09 CATEGORY 05 AD-753080 TR-0073(3240-10)-4 SAMSO-TR-72-277 F40701-72-C-0073290972IR LASER RADIATION EYE PROTECTOR/RESEARCH REPORT, JAN. - JUN. 1971/(IR LASER RADIATION EYEPROTECTOR)Specer, D.J.; Bixler, H.A.Aerospace Corp., El Segundo, Calif. (AG163093) Lab Operations. 11 P. Refs. I JPN. 998ACRYLIC RESINS/ -EYE PROTECTION/ *INFRARED LASERS/ RADIATION INJURIES/ *RADIATION PROTECTION/WATER

B-9

Eye Protection

43 72N21083# ISSUE 12 CATEGORY 05 AD-735799 SAM-CB-71-3 F41609-69-C-OO50 AF PROJ. 7784 000771RESEARCH A.XO DEVELOPMENT OF AN OCULAR LASER PROTECTIVP. FILTER/FINAL REPORT, JUL. 1969 - MAR. 1971/(OCULAR LASER PROTECTIVE FILTER WITH ?RARROWBAND ABSORPTION, LUMINOUS TRANSMISSION, AND OPTICALDENSITY OF 3.5)Woodcock, R.F.; Hovey, R.J.American Optical Co., Southbridge, Mass. (AS850297)Central Resesrch Lab. 41 P. Brooks AFB, Tex. School of Aerospace Med. JPN. 1564*EYE PROTECTION! GLASS/ *LASERS/ LIGHT TRANSI'ISSION/ *OPTICAL DENSITY/ *OPTICAL FILTERS/OPTICAL PROPERTIES/ PLASTICS/ tRADIATION P2OTECTIO.

.~.

U-10

K ~AUTHOR INDEX

(The fir-st two authors of each report only are listed)

Adams, D.O. 13 Lappin, P.11. 19Laudieri, P.C. 30

Basan, F.C. 5 Lund, D.J. 9Beatrice, E.S. 13,23.28Bixier, H.A. 42 Manson, N. 2Borland, R.G. 17 Marshall. J. 24Brennan, D.H. 17 Mautner, U.J. 18Bresnick, G.H. 20,23 Mueller, H.A.8Bruce, W.R. 14Byer, H.H. 28 Payne, W.R. 2

Peabody, R.R. 27

Carver, C.T. 9Chester, J.E. 20 Ready, J.F. 4Clarke. A.M. 7 Rosan, R.C. 29Cleary, S. 7Cordes, W.F. 4- Setter, L.R. 36

Sherr, A.E. 41Dabberdi, W.F. 32 Skeen, C.H. 14Deitz, P.R. 37 Sliney, D.H. 34,39Dunsky, I.L. 12,15,19, Snavely, D.R. 36

35 Specer, D.J.0Ebbers, R.W. 12,16 Spencer, D.J. 15

Egbert, D.E. 10 Stein, 4..26Elgin, S.S. 26

Fallon P.F.Van Pelt, W.F.2Faln PF 3Vassiliadis, A. 22,29

Farrer, D.N. 25Fields, T.A. 25 Walkenback, J.E. 39Fife, W.A. 35 Will'ening, G.M. 38Freasier, B.C. 5 Wolf, E. 6

Woodoock, R.F. 40,43Gallagher, J.R. 30Gibbons, W.D. 10,11 Yanoff, M. 21

Goldman, L. IZweng, H.C. 22

Ham, W.T. 8Honey, R.C. 27Hovey, R.J. '43